Yes, unfortunately it’s not going to be easy for flat rate users.
I would do this, though there may be other ways.
For the rest of your current vat quarter, leave the flat rate as it is, the one you are on now for your sector.
Charge the new 5% from 15th July onwards.
You’ll need to report from the beginning of your current vat period, until 14th July, then from the 15th July to the end. This will give you the 2 figures, but it’s the last you’ll need to apply your new rate to, to recalculate.
This will give you the figure to minus from your vat return by manual adjustment.
Example at 12.5% current flat rate to the new 4.5%
1st part till 14th July turnover is £5000 so £625 vat
2nd part 15th July onwards turnover is £3000 so £375 however, new rate is 4.5% so should be £135
You’ll need to lower vat return by 375 - 135 = 240
Then don’t forget to alter as of your new vat quarter, your new flat rate into quickfile.
Hope it makes sense.