responsive project by mistake

Not in one act, sorry. Non-responsive and responsive are too different. Position properties in a responsive project are mostly set up in % and Auto, whereas in a normal project you always use sizes and X/Y coordinates in pixels. You can try to copy/paste slides, if the size of the normal project is the same as the primary breakpoint view and if you did use the Theme without overriding styles. Try saving the customized theme in the responsive project (if you did customize it). You can use it as a base for the normal project, but save it under another name because otherwise it will not be usable anymore for a responsive project.

You can open the Library of the responsive project as an external Library and using assets from that Library.