Think the Newton is the only modern ship that we could conceivably refit with covariant shields? But it has a warp 7 engine, so a major refit is probably not that attractive. The Keas don't have torpedoes, the Selachii are around 40 years old and max out at warp 7.2 (compared to our warp 7 max cruise). So we don't really have anything in inventory that we'd really want to refit covariant shields to.
AFAIK, we'll theoretically have the option to refit the USS In your face, Time Police with a with a warp 9 core (which is why we went with an early warp 8 core). So the hull has potential to be in service for some time. A refit to covariant shields and all rapid fire launchers something like 30 years from now seems plausible, and a 'last' refit to warp 9 core, type 2 shields, new phasers around 50 to 60 years from now is at least possible.
Point of order, the Keas
do have torpedoes - we didn't fit them initially but they were given a pair in a refit when we made torpedoes standard issue. I would say that Keas (as the budget cruiser) and the Selatchii (as a dedicated combat ship
and one of the most numerous
and smallest ships in our fleet) are actually the
most likely to be chosen for any hasty War Refits.
the Kea, as mentioned, is basically a proto-connie after the torpedo refit, and the ship likely to be filling out the "bulk line Cruiser" role in the coming war (if it indeed occurs), a shield refit would definitely not be atypical at this time.
The Shark on the other hand is
small- one of the smallest ships in Starfleet service, in fact-and we have a goodly number of them; refitting at least
some of them with new shields and possibly torpedo launchers in the leadup to a war would not be exactly a foolish or unjustifiable expense, even if one expected to retire the class as soon as the war's over, since their small size means that smaller yards that can't build modern ships are able to handle them, and the actual installation is simpler and faster.
So ironically the two ships you picked as negative examples are actually the
most likely pair, of all our existing fleet, to receive end-of-life refits!