The first thing you'll want to do is switch to the
Flexi Toolkit V1.47n. In many cases, simply opening the pack with the new version and rebuilding the NXP will be enough to get the pack functioning properly in V1.47n (without warnings). If your pack is also still V1.43n-F compatible, be sure to check the "V1.43n-F Compatible" option on the Settings tab.
The biggest thing to be aware of on the technical side is if you have custom lemming sprites. Be sure to closely examine these sprites in-game to see if there are any oddities; in particular with floaters, gliders, stackers and miners.
Also, be aware that since V1.47n no longer supports secret levels, all levels will be treated as normal levels. You might want to consider (a) whether or not you want to keep these, and (b) if so, how you want to integrate them with the rest of your pack as regular levels.
However, by far the most important thing to be aware of is that
physics have significantly changed. Before releasing and claiming it's an updated, V1.47n-compatible version,
test every level and talisman to ensure they're still possible. You can use the mass replay checker to significantly reduce the effort needed here, if you have a collection of replays for your pack (which you should) - on that note, the new Flexi Toolkit contains a fairly powerful Replay Manager, accessible from the menu bar, which you can use to maintain your replay collection for the pack. The other reason is that, if there's anything you overlooked during the upgrade process, you'll very quickly discover it here.
Finally, when posting your upgraded version, take note that the forum has some special "smileys" used to indicate version compatibility. Even if you aren't upgrading your pack, I strongly advise adding the appropriate one to your existing topics. You can find these icons under the "More" link by the smiley buttons when posting.
This indicates that the pack is only compatible with V1.43n-F (or at least, has only been properly tested with V1.43n-F). This can be used for packs that you haven't got around to upgrading yet, or packs that need V1.43n-F because they use older features that have been removed.
This indicates that the pack is only compatible with V1.47n (or at least, has only been properly tested with V1.47n). This can be used for packs that use new V1.47n-exclusive features, or that may theoretically work with V1.43n-F but you didn't bother to test with older versions (and I suspect that most people won't; there isn't much reason to do so now that V1.47n is officially released).
This indicates that the pack is compatible with (and ideally, properly tested with) both V1.43n-F and V1.47n. This is most likely to be the case for packs that were released shortly before V1.47n was released. This does mean extra effort, so I don't strongly advise making future packs this way - V1.47n-only is generally fine.