We just tore off a cedar shake roof (2/4" to 5/4" rough cedar shake) about two weeks ago. I haven't replaced on here in South Texas in about 5 years.
Here, when properly installed they last a long time. But the shakes and technicians to install them are in short supply. When cedar shake shingles were all the rage here in the 70s, we had them everywhere, especially on the mid to upper priced homes.
Two things happened. We found down here that most cedar roofs wouldn't stand the 100+ degree summers we have here for a usable service life. The game changer for us was the fact that insurance companies wrote exclusions or refused to insure homes with cedar shake roofs as they burned so quickly.
So we change them to a dimensional shingle. You take off all the shakes like you normally do for a roof replacement. Then deck the roof over the skip sheathing with plywood or OSB. Felt it, tin cap it, and shingle. This is also an excellent opportunity to install ridge venting on your roof system.
I am sure in you area of the world you would need to follow the ice and water shield details for shingle, and in some instances it may be a requirement (if not a local code, a requirement for insurability) to install "ice barriers" under the shingles.
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low slope applications, for "dead" (flat) valleys, under complicated tile roof details, etc., we always use ice and water shield. It is cheap insurance for our end product performance.
The last cedar shake roof I put on was about 25 years ago. At that time, the cost to replace with a vastly inferior 0/4" - 2/4" #1 perfection grade split cedar shingle was about the same price as converting to asphalt shingles, even with the decking.
You might weigh your options. If you convert, the next time you need a roof, asphalt shingles are much easier (cheaper) to replace than wood shakes. And if you have "heavies" or the large wood shakes, it is cheaper to replace.
Just my 0.02.