Switch to original labX code (X = 1..8) : git checkout vX
Create your branch at this version git checkout -b branch myX
Merge from your old branch Y (Y = X - 1) git merge tagY
Commit your base code on this lab git commit -a -m labX_start
Do your homework now ...
if need to rollback, do git checkout *
if finished, do not forget to commit git commit -a -m labX_finish
And make a tag for future merge git tag tagX