Edit Garmin coach time objective

No, it will adjust the time frames for the workouts going forward and shall keep the history. You can do it multiple times...

I’m on week 2 half with Greg. 
Ive edited my time more than once and all seems well. Mostly changed it to stop the watch beeping at me as some of the paces are set ridiculously low. I’m very new to running after years of cycling, and my 10k pace was around 6:20. When I used that as my goal for the half my runs were paced slower than 7:30. I just can’t run that slow even at my level, it makes me go all wobbly and I might as well walk! If nearer the end (if I stick with it) it gets too fast I’ll just slow it back down again. 
I also changed coaches after 1 day, and that went ok but took a few syncs to get the watch sorted out.  
and I went from just completing to a time goal and that was ok too.