I've heard it may be tricky and not always possible with the 620 but if you can swap the 27" wheels for 700c you will lose a little of the overall bike and standover height. It may be worth experimenting if you have a set of 700c wheels available.

I have the reverse situation - I picked up a 21" 620 and normally ride a 23". Where do you live? What ever happens you got a great deal on a great bike. What year is it?