Hi, - my 2 cents -
depending on the analysis Tyler suggests, it's necessary to change both. But is there more to it than just changing the valve guides, e.g. valves and seats have to be adjusted and when you start with this the whole head could be refurbuished.
Replacing the seals is not much work, it can be done without removing the head but check here special things for 230 (guides with and w/o nuts, glueing seals).
...WRe