They have switched from NiCad to lithium ion. When new cell chemistries became available they made a lithium+ line. They now have higher amperage batteries with more contacts for their "HP" line of tools. Those batteries and tools are still backwards compatible. I can take an HP battery I bought for my impact wrench and throw it into my 20 year old recip saw I bought at a garage sale and it works.

Ryobi products are low end but they have innovated plenty while maintaining compatibility. Given that TTI has access to the pro market via other brands, and Ryobi geared for homeowners, I don't see this as anything but a win for the average Ryobi customer.