No, you can change anything you want after the fact if you enter a user's profile, or ask them to change it (and provide a guide). I don't like entering a user's profile, or impersonating them... it's a kind of invasion of privacy which I avoid if at all possible. I much prefer to write a guide which walks the user through how to make the change themselves - this takes a bit longer to compose, but once it's done you can point the users to it and say "Here's how". This is a lot faster than going in and setting muted and watched manually for each and every user, and... the more familiar your users get with how to work with the system the more likely they are to figure things out and help each other.
The point is: get your default settings and your guides set up ahead of time so that the users inherit them as they arrive, this saves you time and effort.
If you are doing a migration from a different platform (as we did) your migrated users may well spend a while moaning that it looks different, and the buttons aren't in the same place. The faster they can locate the bit of information which says "On the old system, search used to be here and it worked like this, on the new system, it is here, and it works like this and, by the way here is how you can do a search which wasn't possible on the old platform." The faster they can locate that kind of information the faster they will settle back down to Fighting, Feeding, and Fornicating. Having it in place ahead of time means that you can be fixing any problems, and doing PR work.
Our Guide section is Here. This wasn't in place as we went live, so the first few days were a bit chaotic. I created the category and started writing guides for the most urgent and common questions. Fortunately, I got some buy in from a few users and they helped with creating content, and also just suggesting topics.
I would suggest having the following guides in place (remember, our forum is mostly retired guys and many aren't all that IT literate):
How do I login/logout tell if I am logged in?
How do I access my profile, brief explanation of the sections of the profile?
How do I change my mail preferences, what do they mean?
How do I tell the difference between a public and a Private message?
How do I create or edit a post/topic?
How do I use the composer?
How do I upload photos/files & what can I upload?
How do I flag a post, contact a moderator?
Try and keep the guides short and use step numbers so that when someone can't get it to work they can say "I get an error message when I hit "ok" in step 5"