On variable rate mortgage, I want to change it to repayment what shall I do without re-financing?

I am with mortgage express on the lenders variable rate mortgage, can I stay with lender and go on a repayment mortgage when I'm on a variable rate?
Since I obtained my mortgage 8 years ago my job has changed and my mortgage is on a variable rate- I'm paying interest only.
If I wanted to change to repayment what would I need to do? I'm worried because when I obtained my mortgage my salary was higher but now it's not- what options do I have because I don't think remortgaging will be easy.

Regards x

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