Actually, you don't need any modified pickguards or anything. I'll see if I can explain this clearly...
The pickup in your bass now is a four string
sized Music Man style pickup. It has wide bar magnets that allow it to work for the narrow spacing of that five string. The case is the same size as a four string stock MM pickup. The pickup that you will be installing is a stock size five string MM pickup, which is a rectangular shape with no "ears" and it will completely enclose the four string cavity. So, when you rout for the new pickup any trace of the original cavity will be gone. If you can leave the original pickguard screwed in place while you rout the cavity it'll automatically fit the pickup. As long as the routing is cleanly done you will have no problems at all.
Of course, if you still want to do a new pickguard that is entirely up to you, but it's probably not necessary.
