If you find that after updating an old .hex file (pre October 2018) in the current MakeCode v1 editor makecode.microbit.org the code no longer runs on the micro:bit, you may need to upgrade your firmware.

This is a result of an issue in MakeCode that causes the program to be overwritten when writing data to the micro:bit RAM above a certain threshold.

This only affects micro:bits that are using the 0234 interface firmware, which you can check by opening the DETAILS.TXT file on the micro:bit device. Upgrading the firmware will fix this issue.