Ever found out how commutative ESXi patches/updates are. Possible not all below steps are necessary, but it worked out fine. No lose of configuration data and so on. Started out with build 1117900. Did only 5.1 update 2 and 3.
I was sure I could go from 5.1 update 3 to 5.5 baseline. I found documentation for that - but possibly you could go directly 5.5 update 3. Not a test machine, so I took the conservative route. Let me know if you have been able to skip some of the steps.
Started from the bottom.
|ESXi550-201412001||ESXi 5.5 Express Patch 5||2014-12-02||2302651|
|ESXi550-201410001||ESXi 5.5 Patch 3||2014-10-15||2143827|
|VMware ESXi 5.5 Update 2||ESXi 5.5 Update 2||2014-09-09||2068190|
|VMware ESXi 5.5 Update 1||ESXi 5.5 Update 1||2014-03-11||1623387|
|VMware ESXi 5.5||ESXi 5.5 GA||2013-09-22||1331820|
|VMware ESXi 5.1 Update 3||5.1.0 U3||2014-09-09||2323236|
|VMware ESXi 5.1 Update 2||5.1.0 U2||2014-01-16||1483097|
|ESXi510-201305001||5.1.0 Express Patch 3||2013-05-22||1117900|