The newer ships would probably have been faster and larger.
Assuming they replaced on around or close to a 1:1 basis, the result would probably have been fewer losses as a percentage (faster convoys suffer fewer attacks), but individual sinkings would be of bigger ships. Probably roughly evening out (the actual result would be possible to calculate, but would require all the relevant data..)
(Unfortunately while the liberty ships are probably a good example of the sort of ships that would have been built, they entered in the second half of the Battle of the Atlantic, so data on their losses would have to be adjusted accordingly)
However faster convoys effectively increases the amount of good that can be shipped. Overall, it would mean a lessening of the shipping problems for the allies.