Not sure if you can re-do the VCP550D, but that's the simplest option if you can - ask certification@vmware.com
As for alternatives:
1. 2V0-621D which will give you VCP6-DCV
2. Exams for either VCAP5-DCA or VCAP5-DCD
3. Any of the other 3 VCP6 exams - for CMA, DTM, or NV
Any of those would be better in the long run than letting your VCP expire, assuming you'd like to remain a VCP.
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