Hi Michael McBain. You cannot directly update existing features with QuickCapture. However, you could run a post-process against observations made with QuickCapture to transfer your observations to existing features.
For example, you can first run a Spatial Join using your QuickCapture data and your hydrants. You will want to use the Closest option and set a conservative search radius. See more details about Spatial Join here. Spatial Join will create a new feature layer, which will be temporary. In this layer your hydrants will be joined to the QuickCapture observations.
You can then run the Add Join tool to the original Hydrants layer using the output from the previous step. Make sure you join using the Hydrant IDs.
Once you have that, you can then run the calculate tool to transfer the QuickCapture attributes to your Hydrants and then Remove the Join.
All the above could be run manually from ArcGIS Pro if you need to transfer attributes sporadically, but it could also be automated with a Python script to run it regularly.
Hope this helps.
