My understanding is that an FSRU (Floating Storage Re-gasification Unit) functions in the exact same way as an LNG/LPG tanker, only with a final step that converts the liquefied gas back into a gas to be fed into the grid. This final step is usually accomplished by bolting a re-gasification unit on deck, whilst leaving all of the tanker related equipment intact.
In theory you could convert the FSRU back to a tanker fairly easily, probably in a matter of weeks. The biggest issue you'll find will probably not be the LNG/LPG tanker plant, but rather the propulsion machinery.