I don’t know that there’s a one size fits all implementation checklist. Your business needs and the modules you end up implementing will affect what you need to do. As a start I would recommend you take a look at the user admin guide and other resources on EpicWeb. (ask your account manager if you don’t have access yet).
Having a good consultant on board (or someone with Epicor experience) will be a huge help.
Also, take a look around on this site for posts like this: Making the leap to Epicor ERP - Experiences with support / implementation. There’s quite a few similar posts about implementations and upgrades which will give you some useful information.