Yes, you'd still be able to use the files in Legacy ... but only with Legacy turned on at system/course level. A backup of a course in legacy will bring legacy upon restore of the course. Initially all courses in a migration of the site use Legacy, but new courses created won't use Legacy. One can get a course out of Legacy at a certain version of 2.3, but the process is strange and if one had many courses, very time consuming.
Please see (and follow links from the initial page):
https://docs.moodle.org/29/en/Legacy_course_files
Somewhere in there someone posted an .mbz file and a DB query to run to rid a course of legacy. Haven't tried it myself.
Another way to look at this issue ... teachers will be learning the new system anyway so why not have the teachers re-create their courses (good time to review how/what/why they are doing what they are doing). The files they used in the legacy courses would have to be downloaded from the legacy courses then re-uploaded to their new non-legacy course. During that entire process they are becoming familiar/re-learning with the new version. Perfect opportunity to explore repositories NOT offered in 1.9 (like Google, other).
Would advise working towards removing legacy .... I've asked how long legacy will be supported more than once but no one is responding to that question. No response is a response ... no one knows nor seems to be interested in the 'issue' (to them, a non-issue as everyone should be using 'latest and greatest'). Best, me thinks, to begin working on that now from the git go. An upgrade to higher version (3.0 is near) might bring a surprise when it comes to legacy files - then again, if one digs into the 'fine print' of updates/upgrades one might not be surprised - if there is a footnote or something concerning legacy.
2 cent opinion of course.
'spirit of sharing', Ken