It will send more control data over the WAN Link but that should be it.
The AP will report the information to the WLC and the AP mode (Flex or Local) is irrelevant for the AP ELM Functionality. Just keep in mind that it will add more CAPWAP control traffic, which may only have an impact if you're using a very low bandwidth link. However, it shouldn't be a problem.
You should be ok :)
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