If it comes to light when matching an invoice to a purchase order that the PO has incorrect distributions, you should make the changes at the invoice processing stage (rather than returning to the PO and correcting the error there).
If you feel the PO distributions require altering then:
This will leave the PO status as 'Matched' and the GL entries as required - more detailed guidance is available [1].
The system tolerance levels, used when comparing the purchase order line amounts and invoice amounts, have been changed in the Accounts Payable module to allow invoices with a 5% variance (up to a maximum of £100) to proceed without creating a hold.
Links
[1] https://www.finance.admin.cam.ac.uk/files/ap_mtchtopo.pdf