Although the standard doesn't explicitly require it, many RDM implementations will assume that the response to GET PARAMETER_DESCRIPTION for a given PID will be constant throughout the life of a given responder.
The exact behavior will depend on the specific controllers and responders that you are working with. Some controllers will request PARAMETER_DESCRIPTION for all manufacturer specific PIDs upon initialization, while others will only get the information when needed.
Your best option is to find a way to implement the functionality you need in a way that doesn't require changing the PARAMETER_DESCRIPTION response. You could use multiple PIDs, or some other method.
To give any more detailed advice, we'll need more information about what you're trying to accomplish.
As a side note: Make sure you've read section 6.2.10.2 of the E1.20-2010 document. Manufacturer specific PIDs have to have the same "meaning" across all products from a given manufacturer.
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