Can I change Redeemed Reward Microsoft Points back into Microsoft Reward Points?

Unfortunately no. Once you've redeemed them, they can't be reverted back.

From the Microsoft Rewards Migration FAQ: "any unredeemed MyVIP Gems will be converted to Microsoft Rewards points and can be used to redeem rewards from the Microsoft Rewards catalog once your membership is transitioned. One MyVIP Gem will
convert to one Microsoft Rewards point".

From the Microsoft Rewards Migration FAQ: "any unredeemed MyVIP Gems will be converted to Microsoft Rewards points and can be used to redeem rewards from the Microsoft Rewards catalog once your membership is transitioned.
One MyVIP Gem will convert to one Microsoft Rewards point".

With Microsoft Rewards points, once you earn enough points you can redeem an Xbox gift card to add funds to your account and then use those funds to purchase a gamertag change. The support person you spoke to was misinformed on how many points it takes.
Prior to the currency system Microsoft used points as currency and at that time it cost 800 Microsoft points to change a tag. Microsoft points are not the same as Microsoft Rewards points though so you would need enough points to redeem a $10 gift card as
well as a little extra to cover tax.

For help with the Rewards program, you'll need to post in the Xbox Rewards Program Questions support forum.

The Xbox Rewards program and Microsoft Rewards program are 2 separate programs, so it's not just the name changing. Offers from the Microsoft Rewards program are different. You'll earn Microsoft Rewards points for completing them and can use those points
to redeem gift cards (for Live, Xbox, Amazon, etc.). Any credits you have pending on the program should be deposited this month. Otherwise, they will be transferred to the Microsoft Rewards program as their points.