If its recurring billing the FX rate stays the same in terms of customer pricing, see the answer in this article: Are the same FX conversion rates used to price a multi-month or multi-year term.
However, if, for example, an existing 3-year private offer is renewed for another 2 years, a new private offer will be created for another 2 years. This new private offer benefits from the WMR monthly FX rate that applies when the price of this new private offer is saved.