Yes, BMSE is able to do all the speed change for you. BMS Creator cannot handle the tags correctly, and you will not be able to see how it is done within BMS Creator. If you want to do it only with Notepad, I can explain a bit but hope you understand the BMS file format though.
In a nutshell, use the 08 tag for speed change above 255bpm and not the 03 tag. You need to specify the BPM using the #BPM[xx] tag, using decimal is okay there. Then in put the note(for the speed change) in the measure you need using #[yyy]08:...xx.... |