I don't use auto-increment lots of reasons, but to answer your question, the only way I know is:
1. Copy your table (ORIG.DB) to say COPY.DB
2. Empty ORIG.DB (you did check that the copy worked!)
3. Restructure ORIG.DB and set the min value (e.g. 70)
4. Add COPY.DB to ORIG.DB
NB: ALL the previous auto-increment values will be replaced with values starting at the min value specified (in the above case 70) and incrementing by 1.
I'm not sure if this is what you want to achieve.
Padraig