First of all, congrats on playing the K concerto with your youth orchestra.
No, it is not safe.
1. Depending on what brand strings you are using and the construction of your bass, you may do the bass damage.
2. From my experience of tuning the E down to a D and back up, the string feels wonky for a few days after detuning and it shortens the strings' lifespan.
For rehearsals with an accompanist, better to find one with a keyboard that can be instantly transposed with a push of a button. I know the pianist will complain that it's not "pure" and they'll wimp about the action, but it'll save you 3 or more hours of frustration from changing strings and your instrument resettling.
Change your strings to solos at least a week before your concert so they settle in and and you get used to how they react under your bow. If you have other playing commitments, be a snob and tell them you can't because of some &$^%er invented solo tuning 250 years ago. Or find a bass to borrow. Try to find one that has the same string length as yours or it may mess up the muscle memory of your fingers for your own bass.