You realise that there is an entire thread in this forum about refund or rebooking of cancelled flights due to Covid-19? I suggest you look there for first hand experiences with rebooking after flight cancellation.
To sum it up it seems to be a hit or miss/depending on the agent you talk to whether rebooking to a later date convenient to you without recalculation of the fare /up-fare will be allowed. However, lately it appears that practice has been aligned to reflect what you have been told; rebooking to a later date (outside the narrow window of up to 7 days) is subject to payment of add. fare if P/Z class respectively is not available for the new flights.