Although turnbacks may be frowned upon, there are always exceptions to the rule(s). It's the way you go about it that matters most.

Turning back a varsity game for a higher level varsity game is one thing, but moving up a level should be understandable. College guys get pulled off HS games all the time around here, thus lower-level guys get bumped up to varsity games.

If any of the new varsity dates conflict with your existing lower-level dates, I would try to find tentative replacements. Contact the lower-level assignor & tell him of your opportunity to work the varsity level. Let him know the dates conflict with his, but you have a few replacements for him to choose from lined up for those dates. Then ask if you have his blessings in advancing to the next level.

That way all your bases are covered! You never want to burn bridges because you may not get a full varsity schedule & might need some lower-level games to fill your schedule. The lower-level assignor may become the varsity &/or college assignor someday... you never know.
Just my opinion.

What part of the country are you in that gives basketball assignment before football starts? And why would they give lower-level before varsity? Just curious...