My guess is that they'll probably install a rebuilt battery pack at that point.
Most battery warranties don't require a brand new battery as a replacement anyways. They could theoretically rebuild/repair the existing pack. I found Tesla's terms:
If your Battery or Drive Unit requires warranty service, Tesla will repair the unit, or replace it with a factory reconditioned unit. When replacing a Battery, Tesla will ensure that the energy capacity of the replacement Battery is at least equal to that of the original Battery before the failure occurred. To provide you with even more assurance, this Battery and Drive Unit Limited Warranty will also cover damage to your vehicle from a Battery fire even if it is the result of driver error. (Coverage will not extend to damage that had already been sustained before a Battery fire occurred, or to any damage if the Battery fire occurred after your vehicle had already been totaled.) Your vehicle’s Battery and Drive Unit are covered under this Battery and Drive Unit Limited Warranty for a period of 8 years, unlimited miles/km, with the exception of the 60 kWh battery which is covered for 8 years or 125,000 miles/200,000 km, whichever comes first.
Not sure exactly how this works. Some battery warranties might be from total failure, so obviously they can check the diagnostics to see what the battery capacity was at the time of the failure. But if it drops to less than 70% (Tesla's standard) then I suppose the requirement would be to repair/replace to the point where it's above 70%, but likely where they believe the rebuilt pack should stay above 70% until the end of the warranty. It doesn't sound like a new replacement is required that would restore to 100%.