If you have 1 VM at version 10 and all the others are 4, 7, 8, vmx9 and you really want the 1 other to match one of those other versions, you can try to V2V the troublesome using the VMware Standalone Converter and during the process you will be asked what hardware version you want.
If 5.5 won't let you do that, you can take the hardware that the v10 VM was running on before you P2V'd it and install ESXi5.1 or 5.0 on it and then V2V your VM onto that box. Then shut the VM down and move it over to your production cluster.
That is about the only way to downgrade a VM's hardware version that I am aware of.
You would be much better off going forward though to upgrade the hardware on the 14 others and get used to the web client.