The PHP support in LiteSpeed is the best, you can use php4 for one part of the site, while using php5 for another part of the site, setup is easy, there are a few threads about this on the forum already.
I would recommend converting all sites from Apache to LiteSpeed all at once, instead of one by one. you can verify all site on a different port like 2080 and 2443. If nothing obviously broke, then turn off Apache and let LiteSpeed take port 80 and 443.
Minor issues can be addressed later. should be very few if any. just let us know if need help.