I have been looking for my first seven string for some time now. My local store has an Ibanez UV70P and an Ibanez RG 927qmz. Both of those are premium Ibanez. On the used market i have found a used Ibanez RG 427z prestige guitar for roughly the same price, but it has active pickups in witch i don't wan't on my 7 string. I have heard something that because of the pickup routing on a seven string guitar, changing from active to passive pickups may be alot more work than usual. is that true?
It's no more work as long as you can put up with having gaps at the sides of your pickups.

A few companies are starting to make passive pickups in soapbar sizes though, I know at the very least Bareknuckle are offering their ERG range with the larger casings now.
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Seymour Duncan now offers their pickups in the soapbar sizes.
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Depends on what particular pickups they are. I had a pair of Seymour Duncan Blackouts installed in my RG7321 and they fitted perfectly in place of the passives, no mod required. So depending on what pickups are in that second hand guitar, it might be no issue.

As the other guys said though, you may just have a larger than normal gap either side of the pickups. You could always get a fitted pickguard installed to cover those gaps if it really bothers you, if you don't mind drilling holes for the screws.
For the same money, I'd do the prestige then! Going from actives routes to passive can be done with any pickup, it just might look odd :P

Bare Knuckle Pickups sells 7 string pickups in the soapbar size as well, but they are more expensive than dimarzio or SD options :P
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