As noted in GURPS Traveller: First In, the world-building rules in the original Traveller produced many worlds which, by current science, are impossible. The world-building rules in First In are probably more realistic, but give a narrower range of possibilities. These guidelines allow the conversion from previously published materials to First In.
Population-related details are more or less flexible, and can be ported directly to the new system. However, the physical details of a world can be updated as follows. In terms of a player's experience, the size of a world is not terribly important. It does establish the level of local gravity; but on the other hand, many GMs ignore this. Similarly, a world's hydrographics affect an adventure only a little; a world is wet or dry, and that is all.
But a world's atmosphere is of paramount importance. A breathable atmosphere dictates one style of life, a polluted atmosphere another. People live differently on vaccuum worlds, or on worlds with a corrosive atmosphere. For this reason, we will treat the atmosphere digit of a (previously published) Universal World Profile first.
There are three main types of worlds: Habitable, Vacuum and Exotic. In Traveller terms, a habitable worls has atmosphere type 4-9, a vacuum world is type 0-1, and types A-C (10-12) are exotic. Types 2-3 and D-F are borderline cases.
Habitable worlds, with atmosphere types 2-9 or D-F, must either be Earthlike worlds or else worlds which have been terraformed. In GURPS: Terradyne, Mars is terraformed in only 200 years, and most worlds in Traveller have been inhabited far longer.
Habitable worlds of size 5+ might be Earthlike worlds, or terraformed Hostile (N) worlds. An atmosphere of nitrogen and carbon dioxide could plausibly be converted to a breathable atmosphere within a few centuries, by the action of (possibly engineered) microbes. In game terms, therefore, any habitable world of size 5+ needs no revision.
Habitable worlds of size 4 are almost certainly Desert worlds which have been terraformed. In Terradyne, Mars is the equivalent of UWP 410; after terraforming it is UWP 462. Such worlds should not have a hydrographic coverage over about 40%. Habitable worlds smaller than size 4 are impossible; they should be enlarged to size 4.
Habitable worlds of size 9+ might be of Hostile (SG) subgiant-type world. These worlds take longer to terraform, but compared to the long history of the Imperium the task is manageable. These worlds should have at least standard atmospheric pressure (UWP 6-9).
In First In, Vacuum worlds come in two flavors: Rockball and Icy Rockball. A Vaccuum world of size 0-3 is probably a Rockball. Larger Vacuum worlds are possible, in the outer system, as Icy Rockballs. Such worlds will never have open oceans; any positive UWP hydrographic digit must refer to ice, or to water crusted with ice such as on Callisto. (Cf. FI69) The largest Vacuum world in Traveller should have size 6; this is possible only in the outer system.
The rules in First In generate a great many worlds with exotic atmospheres. Traveller listed three types: A (exotic), B (corrosive) and C (insidious). Exotic atmospheres are found on Desert and Hostile (N) worlds (size 4-9). Corrosive atmospheres, per First In, are usually high in ammonia. These worlds are found throughout the system, as Desert, Hostile (N), Hostile (A) and Hostile (SG) worlds. A small (5-6) world of either type can be reduced to size 4 occasionally. It is possible that a type A (exotic) world was originally type B (corrosive), but has been partially terraformed. Insidious atmospheres (type C) are only found on inner Greenhouse worlds (such as Venus), and must have size 5+. With most Traveller materials, this will not pose a problem. Insidious atmospheres such as chlorine or fluorine, popular in science fiction, are probably impossible, but these chemicals could occur as pollutants in habitable or corrosive atmospheres.
Atmosphere types D-F are breathable atmospheres with exotic shape. Type D is an atmosphere too dense to breathe, but habitable at high altitude. Type E designates an elliptical world, with oceans and high pressure at the poles and high sierra at the equator. (A world can bulge at the equator, but never at the poles.) Type F is the opposite of D: a world with an unbreathably thin atmosphere, which is habitable only in deep canyons. (Larry Niven's Known Space includes a world of this type.)
Atmosphere types D-F are habitable, like types 4-9, and should be treated as above. These atmospheres are rare, and occur only on worlds of size 8+. These atmospheres might occur naturally, or might occur when a Hostile(SG) world is terraformed. For most purposes, treat these worlds as having a standard O2-N2 atmosphere, but reduce MSPR by -1 due to limited land area.
Examples: Rethe (Spinward Marches 2408) is listed as UWP 230. It might be a terraformed Desert world, but should have size 4 or 5. Mora (Spinward Marches 3124) is large, and might be a terraformed Subgiant world. Retinae (Spinward Marches 0416) is probably a Greenhouse world, but if so it should not have any water. Forine (Spinward Marches 1533) is a typical Icy Rockball. Binges (Spinward Marches 1635) is too, but its large size is anomalous. This may be amended to size 6, or blamed on the Ancients.
Daryen (Darrian: Spinward Marches 0627) is probably an Earthlike world, and should be at least size 5. Ordinarily, a habitable world of size 4 might be a terraformed Desert world, but Daryen's extreme antiquity makes this unlikely.
To summarize: in converting worlds from older Traveller materials, any world with an atmosphere of type 2-9 must have at least size 4. Worlds with atmosphere digit 0-1 cannot be larger than size 6; if they are larger than 3, they are probably frozen outer worlds. Type A+ atmospheres normally need no changes; they are probably worlds where life never arose.
First In does provide a somewhat more realistic method of correlating worlds with population and tech levels. This is optional; stone-age people living in a vaccuum is one of the quaint features of the original Traveler. However, the GM who wishes to convert may calculate a world's MSPR and compare it to the population. In infrequent cases, you may wish to reduce a low-tech world's population slightly to bring it within the MSPR. More often, an increased TL is indicated. The vast majority of worlds will need no changes.