So I wondered this at the last update--from v4 to v5--and after much research, AndroidWear is very much like any Android rom on a phone or tablet. Officially, going backwards in an update is impossible using the stock recovery because each OTA only allows for an update from x version to y version and will error if you try to update from w version. The only way to get backwards is to unlock the bootloader, replace the recovery with something like TWRP and flash an old image. Among many problems that you might experience though is that OTAs are not system images in that they sometimes they only update specific partitions. In short, you run the risk of bricking your watch, just as you could with a phone or tablet.
If you can live with it, keep it. If you can't, start doing your research.