Re-grading someone part way through a season can bring a load of issues too. In SWEFTA we learnt the lesson that everyone is regraded at the end of the season based on the scores of the season just completed. To include scores from previous years attaches a legacy and baggage that can be hard to shift, and as has been said above, the system needs to react to a need for a person to be regraded, not for them to stay put.
Personally I would make it just as easy or hard to go up or down. If you do anything else then you'll get bunching of numbers in one grade.
New shooters in our region - or those who don't come with a grade achieved elsewhere, put it that way - go into C grade for 3 shoots, then the scores and comp secs get their heads together and decide which grade looks most appropriate for this person. For their fourth shoot they go into their new grade - and take their first three scores with them.