I just de-tuned my 'ukulele to clean the frets and oil the fretboard. The strings are month and a half old nicely stable Worth CL. After slacking them down and tuning back up they have a whole new round of stretching and settling to do. It's not as dramatic as when they were new, but they did relax and now must stretch again. I can only assume that "pre-stretched" strings would behave in a similar way. For all the pre-stretching they would still need close to a week to settle. So relax, retune, and enjoy the tone changes as the strings settle.
New strings can sound a little dead when first put on. It takes awhile for them to reach their full potential.