I know someone this happened to. She applied for a vacancy within the organisation at the level of the junior.
Boss asked her why, and she responded that she knew the going rate and how much less was required, and that to stay she would need an increased wage of £xxxxx. Worked out well for all concerned.
But take this as a warning, especially if you are female. You do not bargain hard enough.
I spent 5 years setting up a service which I ran on me and two assistants in the NHS. When I left, a man took my post, and told my boss no-one could be expected to provide what we had been providing on such low staffing. So he got upgraded, given 2 qualified staff, and still wasn't covering all I did!
That was the NHS, but I never forgot my value after that. I now bargain hard and can work part-time (tho my needs are few). I work for a charity, otherwise I'd want more.
Women make these mistakes all the time. This really is the patriarchy in action.
OP, listen to this podcast, cos this lassie totally gets it, and shows you how to get what you deserve.
unfuckyourbrain.com/your-financial-value-worth/