Not according to your Win7 Upgrade license agreement. Any previous OSes contributing to your full prior OS license for that computer must be dedicated to that machine.
For example:
Say that computer was originally purchased with a full Win98 license and was later upgraded to XP with an XP Retail Upgrade package and then further upgraded with Win7 with a Win7 Retail Upgrade package (the Family Pack). You must dedicate the original Win98 license AND the XP Upgrade license AND the Win7 Upgrade license to that computer You can't use any of those licenses on another computer while the Win7 Upgrade is is being used on your computer.
If you want to re-use your previous OS then, legally, you'd need to be a FULL license for Win7 (either OEM or Full Retail).