Aside from the multiple concurrent user comment, you're dealing with a continuous form. Code like what you have will only affect one record (always the 1st in the form recordset?) in a continuous form. There is only one instance of your control on the form, right? In which record would you have Access affect the value in the field that's bound to this control? If not the first, you'd have to know the PK value for the record you wish to update. If it is to be all, then you'd have to either loop through the form recordset and update per record or run an update query against the table then requery the form.
I have to wonder why you want to modify one field in one record via code, then run some query based on the change. Why not just let the user make the update? I don't think you've said enough about what you want to have happen, or what the process is, for us to comment about whether or not you're even on the right track. At least, that's my opinion for what it's worth.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.