when I transfer all my files to my new imac, will Microsoft subscription be transferred too?

The answer depends on:

1) your current version of Office

2) the macOS version on the old computer

If Office is an obsolete version in Microsoft's eyes, 2016 or older, it is likely that MS have deactivated the activation servers for it. If so, you cannot install even with a valid key.

This Microsoft link is to their forums like these--much more informative than their support site that makes it hard to stay on Mac-only topics. The only trick is limiting forum views to Mac issues, and that’s easy.

Start here: 

Results in Office - Microsoft Community.

That will display this page header:

Step 1) If needed, use the “Office Topic” pull-down to select your Office app.

Step 2) Limit answers to Mac topics with the “Office Sub-topic” pull-down.

Step 3) If needed, use the “Show Filters” option to further narrow the results.

Everyone there is both an Office user and a Mac user, something you can't say about Apple's forums. I believe you will find the contributors there knowledgeable and very helpful.

Welcome to the Apple Support Communities.

It might. If it is not, re-register it.

If you need more help, contact Microsoft.

Jack

If Microsoft sees it as a new computer, which it is, MS Office activation will need to be re-done.