A very similar discussion already happened on this fourm here is the link
You can upgrade the FMC but, depending on what is out of compliance, you may not have the right do do so under the terms and conditions of the End User Licensing Agreement (EULA). If your FMC is licensed and under support then you have full rights to upgrade.
Any managed devices with out-of-compliance Smart licenses will not be able to deploy any policies that may use the unlicensed features. Even if you make no change to the policies that use those features, it is recommended to sync policy etc. from FMC to all managed devices following an upgrade.
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