when i did mine i had to leave it a couple of days between taking the old lens out and putting the new one in as my fingers where aching so bad....the time also comes in handy for calming down before you smash the lens frame into 10000 bits
i took the lense out this morning which i found quite easy probably because i had the strength of the built up anger made by havin exams today and the pure difficulty of takin the lense out