I reverted back to 8.1 using WDRT, and then using iutool.exe to install the CABs listed at the top of this page to get to the RTM release of Windows 10 Mobile. After that, I tried a few different "next steps" and ended up with very different results:
1) I allowed normal OTA updates to run, and after two successive OTA updates, I eventually gained access to the 2018-04 AU build (14393.2189).
2) I reset back to 8.1 again, and followed the same initial steps, but instead of allowing OTA updates to run, I kept WiFi disabled, and instead I tried to follow the instructions linked here (
https://forums.windowscentral.com/w...test-10586-xxx-14393-xxx-15063-xxx-build.html) to directly upgrade to the first release of CU (15063.251) using a newer version of iutool.exe. It failed.
3) I repeated the reset steps again to get back to Windows 10 Mobile RTM; then, I used the newer version of iutool.exe to upgrade to the final directly downloadable version of 14393 listed in that thread, as an intermediate step to upgrading further to 15063.251. This time, I was able to get to 15063. When I then tried to do an OTA check for updates, it detected the fact that the 2018-04 update was available, but the download quickly failed. I repeated this multiple times over a couple of days, but the failure was consistent.
I've gone back to the results from attempt (1), because it leaves my phone with the most recent set of security patches. However, I'd really like to know if there's anything I should do differently to successfully get myself on to the 15063.x builds. Any advice?