Hey there Alice,
Thanks for reaching out! Our bike shops are authorized to work on Tern ebikes, however, you'll want to reach out to the West Hartford, CT store to check in with the techs there to check with them about what work you would like done. If you're referring to the limited warranty offered by Tern, then it should not be affected. If you are referring to a warranty offered by another store (not REI) then you would have to check with them about what may or may not void the warranty.
Hopefully this helps, thanks!