The difference is the dosing form - one is controlled release (results in longer duration of action) and the other is normal release. Where I've been working we've been faxing the physicians and asking them to just switch everyone to generic paroxetine and they have complied, usually shooting the dose up to whatever the next highest available mg (i.e. paxil CR 25 mg --> paroxetine 30 mg.) Some have switched from qDay to half the dose BID. Nobody has complained of having crazy anxiety attacks yet, so I guess it's been ok.
And I rather like the Zpak, from a retail slave standpoint anyway. Just grab the green box, slap a label on it and you're done.