Can you interchange batteries in power tools that have the same voltage but different ah levels?

As long as the connection is secure and using the battery causes no safety hazards (i.e. no visible leads, properly secured connections, the battery was designed to fit the specific power tool) there should be no issue.

AH or Amp-Hours is a measure of the length of time the battery will provide power before dying. Higher numbers mean a longer run time between charges.

The voltages must be the same to avoid damaging the tool.