(Sorry for the long response time; my employers have been getting their money's worth this week.)
I know that it is possible; Federico did something like that in his plugin (plugins?) I imagine features can only be disabled at load time, not on-the-fly; though I might be mistaken there.
On the other hand, replacing built-in functions is the whole idea in this plugin. If there is some serious incompatibility, I'd rather fix it than invent workarounds. Are you finding some problem, or just squeamish about replacing Rex's code with mine? (Can't blame you for that!)
Offhand, I think my preference might be to enable more functions in the QCAL plugin, rather than changing ISO8601. At the moment, QCAL only supports those new functions which would be useful for user-defined holidays; but that could change easily enough.
Charles,
First, no need to apologize for the work-induced delay. I suspect everyone here understands that, but thanks for saying it.
I agree that my goal would be to completely replace Rex's functions with yours, but there are some undocumented incompatibilities that I would be hard-pressed to describe as bugs. They're more like different undocumented behavior for the same not-quite-documented syntax.
I need to use the original functions until I get all the discrepancies in my coding resolved.
1. Rex's functions accept a string of spaces (or tabs) as an equivalent for a comma in separating arguments in functions. The ISO8601 plugin's functions do not, and simply concatenate two arguments into one, causing a parsing error. Cf. the two versions of @MAKEAGE[%_DATE %_TIME] . That was lousy coding on my part, but I've got a ton of 'em and until I get them fixed, I need to use the original functions.
2. Error messages appear not to be written to the file specified for error output in the .INI file LogErrorsName directive. I specify mine to be the same place as the LogName directive; this makes it easier to find errant coding by examining the log.
3. I found at least one occurrence of a function requiring a final parameter where Rex's version does not require it. Unfortunately, I didn't make a note of it and don't remember which function it is. (When I find it again, I'll report it.)
Not a problem, but the HTML page has a misspelling "Regualar" instead of "Regular".
Would you consider adding a new date conversion (maybe "@CONVDATE") in which you can specify both an input and an output format? The usage would be something like, on a system using format 2 default, there is a format 1 input, and you need output in format 4.