It is really not me that says that, but some OEMs. Probably it would always be better to install a system with the correct controller. But I have changed my systems on some occasions without problems.
Did you find a place to show you how to change them in Windows 7?
If the SSD was already in the system, was it being used for something else, like Rapid Start? Does it show up in the Intel Rapid Storage Technology as being available...? Some options may be in the Bios for such things.