If you want it because it's the trendy size, then I'd say no, especially as you'd have to lose two cherished bags to make it happen. If you feel the size would be perfect for you and it's a bag you'd use a lot, then do it.
Weigh up the likelihood of ever replacing the two bags you'd have to lose to get a 25, this might be a difficult task and you can't replace sentimental value. If this is something you'd have no regrets doing and the 25 is the size for you, then do it.
Also if you decided the 25 wasn't right, you shouldn't have any trouble selling it for the foreseeable future.
Just a few points to ponder.
Weigh up the likelihood of ever replacing the two bags you'd have to lose to get a 25, this might be a difficult task and you can't replace sentimental value. If this is something you'd have no regrets doing and the 25 is the size for you, then do it.
Also if you decided the 25 wasn't right, you shouldn't have any trouble selling it for the foreseeable future.
Just a few points to ponder.