If I stop working with one of my suppliers, what happens to the bookings I already made through them?

The bookings stay valid, because they are held by the supplier and the hotel or airline, not by your website. Ending your arrangement with a supplier stops you making new bookings through them, but the reservations already confirmed remain live and your travellers can still check in and travel on them. What changes is how you manage those bookings from that day onwards.

Once the connection is switched off, your portal usually keeps showing the booking as a record, with the reference, the traveller names, the dates and the amount, because that information was saved when the booking was made. What it can no longer do is talk to the supplier for you. Cancelling, amending a name, changing a date or pulling a fresh voucher may stop working from inside your own screens, and your staff will have to do it directly with the supplier instead, usually by email or through the supplier extranet. Whether anything at all still works depends on the supplier and on the reason the connection ended.

Refunds are the part that catches people out. Money for a cancelled booking normally flows back through the same supplier account it was paid into, so if that account is closed or in dispute, a refund your customer is expecting can sit still for a long time while it is sorted out between you and the supplier.

Before you switch off any supplier, list every booking with them that has a travel date still in the future, agree in writing with the supplier who handles changes and refunds for those, and keep your access to their own booking screens open until the last of those trips has finished.