Why the R class could not be rebuilt like Renown

How easy would it have been to modernize the R class along Renown's lines?

It would have been difficult because the R class lacked the internal space that Renown gained by omitting a main-gun turret.

Internal volume was the main obstacle to rebuilding the R class along Renown's lines. Removing a turret from the Renown design freed machinery and arrangement space that could be exploited during reconstruction. The five-turret R-class layout lacked that margin. A similar transformation would therefore require much more intrusive work.