I understand you have some questions regarding exporting and importing your users’ hashed passwords from one tenant to another.
Yes, that is correct. You will need a paid subscription to request an export of your user’s hashed passwords. Importing does not require a paid subscription. Check out our How do I export passwords hashes? FAQ.
Both are allowed. Those users can continue to use their current passwords provided they were imported with their hashed passwords. And they also have the option to use the “Forgot Password” feature.
Unfortunately, I do not know how long it would take for this request to export your tenant of 300 users’ hashed passwords. However, I would not expect it to take long at all.