You need not pay or update. The release that you purchased never expires and the ability to use mp4 and all features remains unchanged.
Updates are not mandatory. If what you have works well, stick with it. Nothing will be disabled.
Unlicensed versions can always be test driven. They are trials until licensed.. Some features may not be available after a trial period. There's also a free version, with basic features..
If mp4 fails, AVI usually succeeds. Or a reinstall of your older version will enable it. Install of the latest release on the NCH site may as well.
If you take the time to provide needed info, the peer-support folks here will help work around issues and locate a version that was released within your six-month license window. You won't be out any money and the program won't be hobbled.
Many editing, financial and other programs are now offered on only a subscription basis. NCH has that option as well. If you want unlimited upgrades at no extra cost - but for the subscription fee - then that's the way to go.