Yes, it's understandable. 4PSA has also had to rewrite all their addons as well. This happens everytime there are significant changes to a software, although I personally think that Plesk should have bumped the version up like to 7.6.0 instead of just the .3 to .4
7.5.3 to 7.5.4 should have been just 'a few minor bug fixes', but with the number of additional changes and recoding the interface to PHP5, that should have been enough to warrant bumping it to at least 7.6.0 (my opinion)