The first tag cannot be changed once you publish and if you post to dtube then dtube will automatically be the first tag.

Your post will still qualify for Open Mic, but when openmic is not the first tag you'll miss out on votes from all the community that follows Open Mic.

Here are the rules:

https://steemit.com/openmic/@luzcypher/steemit-open-mic-week-128-record-your-performance-and-win-steem-sponsored-by-pfunk-and-luzcypher

and a little more about why we have rules.

https://steemit.com/openmic/@luzcypher/what-are-the-rules-to-enter-steemit-open-mic-and-why-do-we-have-rules-anyways

https://steemit.com/openmic/@luzcypher/steemit-open-mic-loves-dtube-so-why-do-i-have-to-add-a-youtube-video-to-my-open-mic-entry

Thanks for playing and hope to hear more from you.