moving from textile to html (redactor)

I have a client with a blog who has been using textile with markitup. They have asked about using a WYSIWYG editor for posts.

I wondered about the implementation as changing the template will effect older posts as well. I have tested a template locally using both - I changed editor="markitup" to editor="redactor", added html="true" and kept "textile="true". This seems to cope with the old textile formatting and the HTML tags redactor adds.

Is there a possibility of conflict here with keeping textile on while using redactor?

The original template has:

<perch:blog id="postDescHTML" type="textarea" label="Post" order="4" textile="true" editor="markitup" size="xxl autowidth" required="true"  imagewidth="500" imageheight="500" imagequality="85" />

The new template I have been testing has:

<perch:blog id="postDescHTML" type="textarea" label="Post" order="4" editor="redactor" textile="true" html="true" size="xxl autowidth" required="true" imagewidth="500" imageheight="500" imagequality="85" />

Although it seems to work I don't think you're meant to use both together and I'm worried about breaking something by doing this. There are over 300 existing posts so it's not really feasible to edit out textile on all of those.

The other option I thought of was having two different post templates (one with textile/markitup, one with html/redactor). Is there a way I can make the new default template html/redactor while setting the old posts to use the textile/markitup template (without going through each one manually)?