We're migrating, what can we do to prepare?
Okay, your library is migrating to Evergreen, and you want to know what you can do to help that process? Presumably your library or vendor has a project plan for the technical aspects of migrating the data, running the system, etc., and we'll put that outside the scope of this discussion. What else can be done to smooth the way and make your life better?
Weed your items, clean up your bib records, and re-think your policies
Change can be scary, but if you're going to change, try to change as much as you can for the better. The Evergreen catalog gives you a lot of ways to search and explore your bibliographic data, and any new ILS might expose your data in unflattering ways, so work to clean it up! This is especially true if you're joining an existing system; you don't want to be the one to "pollute" a good database. You should also revisit your library policies and workflows, to see if there are any kludges or workarounds that your legacy system has forced on you. When learning Evergreen and asking for changes, the temptation will be there to grandfather in your kludges and workarounds. You shouldn't do this. Often, you'll ask the wrong question, and you should really take a moment to figure out the heart of what you really want to know.
For example, you might ask: Does Evergreen delete brief bib records (created by a "Fast Add" process) on check-in?
The answer is no, but that answer would mislead you. Your real worry might be about brief records polluting the database, but Evergreen doesn't create brief bib records in the first place. For fast-add or "pre-cataloged" items, it creates item records that are unattached to bib records. You can later "catalog" that item by transferring it to a bib record, preserving its circulation history.
Simplifying your circulation rules will also make things easier for you (and your patrons). Evergreen can conceivably let you determine circulation behavior for any of your libraries based on almost any data in the system, including time of the day, random numbers, or the patron's last name. So you can make things as complicated as you want. But do you really want things to be complicated? Has your legacy system forced any complications on you, such as the overloading of orthogonal concepts into a single Item Type field?
This leads into…
Familiarize yourself with Evergreen; don't wait for training
Play with the community demo server, join the mailing lists and ask questions. The more you can learn now, the better prepared you'll be during training and during go-live. The Evergreen developers are also on those lists, and not hidden away in the bowels of some corporation, so seek them out! You might find needed changes and gotchas ahead of time.
Be involved; take charge of your destiny
Be pro-active. The Evergreen community is a mix of developers and users, and your time and domain expertise is a resource we all value. Evergreen adapts to the needs of the community, and if you're a part of that community, it will adapt to you.