Yes, a cam change will be necessary, since the lobe profile is different on solid lifter and hydraulic cams. It's theoretically possible to change a cam with the engine in the frame, but it's not something I'd be willing to try. Finding a suitable replacement cam right now is going to be a challenge, since most of them are "out of stock" at all the vendors I've checked recently. I'd suggest finding a cam regrinding service and getting a solid lift profile ground onto the cam you're running now. There's usually some new solid lifters available on Ebay.
Recently, I found a man near my shop who has a cam grinding machine and 50+ years of experience. I had him put a Melling CCS-1 lobe on a 235 cam, and it looks like it's brand new. Drop me a PM and I'll share his contact information with you. He's 90+ years old and currently on chemotherapy, so I don't want to publish his contact info on the open forum.
Jerry
