Is it possible to change e.g. four consecutive quarter notes into four consecutive eight notes (and vice versa) WITHOUT affecting notes already written in the succeeding bars?
(There s a few different ways of doing it, all of those involving “Insert” activated, which makes quite expected - but at times undesired - adjustments to succeeding notes.)
My method:
Select the notes and push the shortcut for eighth notes.
INSERT: Insert a dotted-quarter rest after them.
INSERT: Delete the rests
There is probably a better way…
In many cases it might be easier and faster to reinput. Otherwise you could copy to a scratch staff and edit there, and then paste back.
Or, in the case of making the phrase shorter:
Select the phrase, with insert mode on change to 8-notes, hit R for repeat, turn off insert mode, press delete.
Making it longer:
Select phrase, R for repeat, turn on insert mode, select phrase, change to 4-notes, select the just repeated phrase, delete.
Agree on the reinput remark, this is certainly a minor - if any at all - annoyance. Great workaround btw! 
LAE’s method is really cool! Remember that if by chance this is something you would need to do a lot of, you could record a macro using the Script menu in Dorico and invoke it once you’ve highlighted the phrase. Works like a charm! I wish Dorico could let a user save and name Macros. It’s limited now to just one. 
This could be a great convenience to users!