Can't trim snowboarding activity?

Eta: wearing a Vivoactive 4 with Connect for Android.

Thanks. I had seen that, and initially it didn't seem to work, but got there eventually.

This makes sense to me, but does changing it to running lose any of the data?

no

I tried exporting the tcx file and importing as new. Then set that one to running and it still didn't allow me to trim it. And I accidentally got the half marathon badge too which I'm not happy about. 

Trim won't work with an imported tcx file. Use fit file format, it will be the one that says, "export original."  It will export zipped, so you have to unzip it to import it back into Garmin Connect. Also, you'll lose the half marathon badge as soon as you change it back to snowboarding.

Thanks for the solution.. but can I say it's simply stupid not allowing trimming a skiing / snowboarding activity from the Garmin side?

Did the same mistake and managed by the same solution! But how can Garmin have missed to add that opportunity!