Switching from stable to current is causing challenges

I used to do this all the time without much difficulty, but the the last few weeks, I just can't seem to upgrade from Slackware64-15.0 to current without bricking the system. The usual difficulty is with glibc, so I upgrade that separately before anything else, and that worked fine. But when I run upgrade-all, a whole bunch of new problems show up, such as missing libicui18n. So I rolled back icu4c to the stable version. But more problems kept cropping up, and the system became unusable. The only option is to re-install Slackware64-15.0 from scratch. I am wondering if there is a specific sequence of packages that need to upgraded, other than glibc before doing upgrade-all.