Call BMW FS to discuss. I don't think your payment will remain the same if you extend the lease long term. That's what they told me when I asked them last week at least... I'm not close enough to lease-end to get exact numbers, but the BMW rep said that the extension would be at new terms. He went on to say that they would not be favorable. All that being said please report back with the deal that you get from BMW FS.
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports