Windows 7 Family Pack is 3 - Retail Upgrade licenses which can be transferred to new computers, as long as the computer you are removing it from is no longer being used (hard drive wiped). You cannot at anytime have more than 3 activated. You can transfer the license to another computer as long as the computer you are moving it to has a qualifying OS that can be 'upgraded' to 7.
Your question is confusing though. If you upgrade/replace one of those computers (assuming you mean ones you used the key on before), you are not transferring the key. If you replace the motherboard you can reinstall the OS as it is a retail OS with keys that can be transferred.