You can not upgrade ESXi to ESX, so you can:
1. Upgrade ESXi 3.5 to ESXi 4.0
2. Do a fresh ESX 4.0 installation
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VMware vExpert '2009
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EMCCAe, HPE ASE, MCITP: SA+VA, VCP 3/4/5, VMware vExpert XO (14 stars)
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