This procedure has two parts.

Part 1: apply single master to the spread page

Apply single master to the even page you want to convert to a spread. Note: the content on the right side of the former spread remains.

Part 2: move the content from the spread to the next odd page

Select all content on the right-side of the former spread, cut it to the clipboard and then save the spread.

Open the odd page that has to receive the right-side content from the spread.

Paste the content. Note: the content is pasted on the pasteboard, but it remain selected after paste.

Drag the pasted content to the page.

Save the page.

See also

Convert two single pages to a 'fake' or 'real' spread