If you are using the mailing list (pywikipedia-bugs) to track bugs you don't need to worry, All of the phabricators e-mails will go through the old mailing list and won't change for you. If you worry about bugs that you are cc'd and maybe you'll lose it the only thing you need to do is making an account with the same e-mail in phabricator (even you can add your e-mail address in an existing account phabrictor supports multiple e-mails for one account).
If you worry about how you can report bugs, there are several helps like this session
. but TLDR (maybe TLDW, dont watch): when you want to file a bug there is an option named project, Add "pywikibot" and it will be in pywikibot bugs.