Halfway through changing rear gear out cable
Yes, it's ok.

You'll be having the largestsmallest cog engaged, that's all. What you can temporarily do is set the limiting screws to the gear you wish.
Or you can have a short "pigtail" inner cable fixed somehow so it keeps the rear derailleur on the gear you wish.
Or you can take a punch or a small (Philips head) screwdriver, and a file and work the outer cable out.

Before purchasing the proper cable cutter I used to cut the outer cables with electric cable cutter, bringing it back to shape with a Philips head screwdriver and making it straight wit a table grinder. It gave a perfect result every time. Only the proper cable cutter is much faster.