2 of my 3 richdel anti-siphon valves are not working, and also it was installed not high enough in the first place, so I plan to replace them with rainbirds and also elevate them for the backflow protection to really work.

I'm a newbie. The existing system is vertical PVC pipe + slip/male thread adapter + richdel valve. what do I need to do?

It looks like I have to cut valve and adapter off together, hook a slip/slip to the vertical PVC pipe, then add in an additional 6 inch pipe, add another slip/male thread adapter, then hook to the new valve?

when I slip the pipe into the slip/slip adapter, do I need to put any glue in or simply push-in is enough?

The exiting valves are squeezed into very small space, surrounded by the driveway and a shrub, which makes it very hard to dig deep or cut it.