Can I substitute a 100 amp circuit breaker in a 90 amp spot?
Depends on the label on the Heat pump, the wire size and type, and the stab rating on the panel cover.

Breakers do wear out, it would still be a benefit to replace with the original size it if was the correct size to start with.

It is rare that anybody installs a smaller than maximum allowed breaker at the time of installation, but the information you need can be found on the nameplate of the Heat Pump, it should look something like this:



The line that says "max fuse amps or hacr type circuit breaker" is the line that tell you the maximum breaker allowed. This breaker can be at the electrical panel or at the disconnect at the unit. If the breaker at the panel is sized to the maximum listed then the wire only has to meet the rating of "minimum circuit ampacity". But if the over current protection at the unit provides the maximum allowed breaker or fuse, and the breaker in the panel is larger then the wire has to be treated as a "feeder" must be matched to the breaker in the panel. 

You can see in the label shown that the breaker is allowed to be 50A, but the wire has to only be sized for 29A. But if you feed it from a 60A breaker in the panel and a 50A breaker or fuse at the unit then you would need to up the wire between the breakers to 60A wire. The amperage rating of the wire is based on insulation and conductor type from a chart in the NEC (with additional restrictions on wire #10 and smaller) and the temperature rating of the terminals where the wire is attached.

Then there can be an issue with the breaker in the electrical panel. Many panels have a restriction on the label that limit the total amperage of breakers installed head-to-head. If increasing the breaker size exceeds this number then you can't do it.