How can I keep my beam-grouping when I change the meter to open? I have a long solo piece with fixed beaming and changing meters but I want to change the piece in order to have no meter but the same beam-groupings
Unfortunately, you can’t at the moment, because unless you have already explicitly set all the beaming by using the various commands in the Edit > Beaming submenu, then Dorico will re-beam everything according to the (default) beat grouping of the open meter. In future you will be able to specify the beat grouping for an open meter, but I’m not sure when this will be possible.
In the meantime, it might be more practical for you to employ a fiendish workaround, e.g. to hide your initial time signature, set the default barline type (on the Barlines page of Notation Options) to dashed, and then set the gap between dashed barlines (on the Barlines page of Engraving Options) to a very large number.