We did this with LMT in July. Bought 4 day PH tickets then upgraded. We saved $44 on each ticket. That savings was bridged so we paid $575 for a Deluxe AP instead of $619. We were able to use our MM before upgrading, we we lost out on a little bit of discounts (probably only $20-30 for the few days before we upgraded, so not a big deal to me.) MP had not started when we were there, so if you plan to upgrade to the annual $75 AP MP, paying $10/day before your upgrade might eat up your savings. Ideally, if your first day was your MM and you didn't have to worry about block out dates, you could upgrade right after your MM.