They should try to make as valid a story as possible if they want to please the fanbase, but some retcons are inevitable. Just make sure they're minor.
- The Republic and Empire have cooler looking ships? Go right ahead!
The Second Imperium crisis happened earlier? We'll get used to it.
There was no New Jedi Order? Not gunna happen!
Palpatine wasn't cloned?
Kyle Katarn didn't exist? I disapprove.
And so on.