I switch out my 12-27 for a 12-34 (I put a XTR cassette and derailleur on the rear) and I did need a new chain - the original was just not long enough. If you keep the longer chain on when switching to the 27, then it will be too long and you will have chain slack. There really is a big difference between a 27 and a 34 not so much if you are switching out a 27 for a 25 or 23. Suggestion - have a chain for each cassette. Use a Conex link so it's easy to remove and replace. When you are using one you can be cleaning hte other. Probably not a bad idea. Also you will need a different derailleur for the 34. The regular Ultegra or whatever you run just isn't long enough. You need a mountain bike or touring derailleur.

Note: I put the XTR on3 years ago for my Utah trip but never replaced it with the 27. Turns out I like having the 34. Are you sure you will be changing the casettes out?

Also to change out a cassette you need two tools - a chainwhip and I forget the name of the tool that goes in the middle. One holds the hub tight , the chainwhip loosens the cassette. I would got o your LBS and ask them to show you how to change out the casette and what tools you need. Neither was particularly expensive.

Also there is a cheap tool you can buy that measures chain wear. I think wear varies for each rider and how hard they mash their gears.