Can I convert an old square d qo panel from a main lug panel to a main switch without doing a service change?

can i convert an old square d qo panel from a main lug panel to a main switch without doing a service change?

Not sure I understand the question, but if you want to backfeed the buss through a double pole breaker and use it as a service disconnect, the answer is NO.

1. The main breaker must be a bolt-on type, QO's are stab-on.
2. The panel must be rated "Suitable for Service Equipment" by UL or other approved testing laboratory. (It has to have a factory installed Main Bonding Jumper)

This can be done with the proper breaker retention device, but I doubt it would be available for an older panel.

Are you sure it is a main lug panel?? I bet it is a split-buss panel.
Is one of the two-pole breakers marked "Lighting main"?