I have an existing page layout that I want to slightly modify and use. When I upload the modified file to a pages library it works fine but I would like to be able to select that page as a layout going forward, is there any way to do this?
Instead of uploading the page to
page library you should upload it to
Master pages and page layouts by going to
Root Site > Site Settings > Web Designer Galleries > Master pages and page layouts
Or navigate directly to
Note : Not all ASPX files considered as a page layout so you should make sure that it's a page layout page as you mentioned in your question.