Hi @Matt Sullivan, phew, that's a lot to unpack!
I have to ask "are you sure?" Before going further. Is there another PM company that might be able to offer pricing more in line with what you need to be more profitable? That could potentially save you an enormous headache. 80 doors is a lot of doors to manage on your own.
If you insist, I would not use Buildium, but use Propertyware (both owned by Real Page) - Propertyware is more robust and more customizable.
1) Your transition plan has to have a checklist for each door. You have to get the tenant's information, their payment history, any relevant lease notes, repair history on the door including open work orders, all into your management software. You need to enter that data 1 door at a time into your new system.
2) Make sure to coordinate the transition with your current PM, right after rents are paid. The data entry part will take longer than you might imagine. I suggest with 80 doors, making sure you have 2 escrow accounts, 1 for security deposits and 1 for the draws to yourself or the entity that owns the doors. Your property management software will make this easier, but you will need to do monthly reconciliations to make sure the numbers match (I strongly encourage outsourcing this part.)
Make sure you have the security deposits from your current PMC that match up with your leases. This should be part of step 1, your spreadsheet should have a column for this number so it can easily be added.
Make sure you get the contact information for any vendors doing/have done work on your 80 doors; particularly any ongoing maintenance, so you can enter those vendors into your new system.
Make sure your current PMC notifies tenants of the transition of their security deposit and of the management of the units. Make sure the PMC gives out the number/email you want to be fielding calls/emails from. Highly suggest a dedicated line and email address.
After the current PMC notifies the tenants of the transition, you will notify them of the transition. This is where I would suggest making sure you have up-to-date contact information for all your tenants; and if possible, get all of them to pay rent online via the tenant portal (most software has this functionality.)
Last, pay attention to lease end dates. You need a process to make sure you're not letting all the doors slide to m2m. If you use a software like Propertyware, it's easy to run reports monthly with leases coming up for expiration, and then to track where you are on the renewal/non-renewal process. It would be terrible to do all this work to save money, only to lose it on vacancies that sit on the market. Good luck!