I activated my brain and found (in 10 minutes!) hopefully the best solution for this special purpose. The following macro does, what you want. There are some special extras included.
[ ^t^p]++ deletes spaces, tabs and line breaks following the comment, if they exist. This avoids also creating a empty line instead of a multiline block comment.
The IfFound command after "-->[ ^t^p]++" is needed to exit loop at bad commented html files, where the comment end is missing (Frontpage sometimes creates such bad comments), letting the cursor at start of this bad comment.
The macro can also be use for replacing single line html comments.
I am incapable to write macro. That is because despite I have read the guide. I didn't succeed to write macro and to modify that you are written.
(for example to add "the replace of page break" I can only that it need of add the string "^B" but making this, the macro doesn't work)
For this reason I ask you: if you know a website where try example of macro or if you know a guide (not the guide of UltraEdit that is little explicative in respect to this).
I certainly will have new exigency and I will have need to make new macro.
UE means a single, normally not visible character with hexcode 0C. That's not a <br>, <p> or <hr>!
I don't know, if a guide for UE macro exist. I never needed one. I simply learned it by reading manual, recording macros and modifying the macro code after record. My first macros and regular expressions were also very simple and not so complicated as this one. Learning by doing is always the best method. But do not start with a tricky one. First you have to learn to crawl, before walking or running!
I have planned to create a collection of my best and most useful UE macros with detailed description and some test files for every macro. But at the moment I have no time to spend a lot of hours for creating this collection. So the only advice I can give to you at the moment is, read as much posts in the macro and find/replace forum as possible and try to understand, how the works.
Best regards from an UC/UE/UES for Windows user from Austria