LifeCaulk/3M 101 are polysulfide caulks. I've been using LifeCaulk for about 40 years and have never been disappointed. It remains flexible and maintains a seal practically forever. I've pulled up fittings after more than 30 years and the caulk was still in great shape, bonded and pliable. Polysulfides also seem to keep in the opened tube a lot longer than 5200/4200 polyurethane sealants. I also find LifeCaulk a little easier to work with. It seems to spread and cover easier than polyurethanes. I use paint thinner for clean up which works way better than Acetone. Haven't tried alcohol since paint thinner works fine and is cheaper.

Polyurethanes are billed as adhesive caulks. In the case of 5200, consider it glue. If you use it, be really certain that you don't want to remove the fitting for a long long time. There are solvents available for removing 5200 bedded parts but I don't see the need for the adhesive qualities in the first place. I've also heard it rumored that polysulfides harden and cause the integrity of the seal to fail after long term use. No personal experience with that happening, however. Have heard people say that they've gotten a seal with 5200 on things like chain plate deck areas where other caulks, including polysulfides, haven't worked.

Polysulfides are a mild adhesive so can be a little challenge if you have to remove the part. Slow steady pressure stretches the stuff and eventually breaks the bond. I like to use chisels/wedges driving them in a bit and then letting them set for a bit. If you can get a thin wire under a corner of the fitting, the wire will cut the caulk. Also use the tension of a line, like on a winch, to pull a fitting off.

I don't know if I'd panic about replacing the hatches. Hatch openings are pretty much standardized among manufcacturers. Don't think Lewmar would shoot themselves in the foot by making a new line of hatches incompatible, size wise, with an older line. Of course, we've all known manufacturers who've not only used a .22 but a 12 gauage shotgun to blow off extremities. In Lewmar's case, if they are changing their standard sizes, it could be an issue of conversion from from Imperial to Metric measurement if the hatch standard was initiated before England went metric. I'd check with other hatch manufacturers to see if they make a size that will fit.