How about the other 2, SM and FM? Are there any changes?
Honestly, there's not much change in methods - you could probably forego that one, but you'll need to borrow the last chapter off a friend.
Specialist has had a shit ton changed, though, and about 1/3 of the book will be different. Not so much in new content - a lot of it has been moved up from methods, but there is some new stuff in DEs and the new statistics area of study, so you might be able to get away with borrowing from a friend and learning some things from the old methods book, but I wouldn't recommend.
Further... I'm no expert, but you miiiiight be able to get away with using an old textbook. The biggest change there seems to just be formatting (instead of 3 optional modules, there's only 2, and a second core module of business+series was made). But just like with specialist, I'd try with the new textbook as much as possible.
Having said this, I know what it's like to not have that kind of money lying around, so if you do go with the old ones, just do your best to keep up with friends what you're missing - maybe buy the old books cheap and photocopy from a friend where you have to.