I could try to do a screenshot later but there are actually several catchup entries: SEL, MEL, IMC, Day, Night, different types of a/c etc
I forgot the actual ifr entry and could not find a way, 8 months later, to add a new catchup with the original date (again, for cleanness sake).
For now, I edited the SEL entry to add the IMC
Mark did my product support for me here (thanks, Mark!). Indeed - when you created a set of catch-up flights, it's just a set of flights like any other in your logbook.
But I'm not quite following your scenario here: if you used the MyFlightbook starting totals tool, then it only created catch up flights using different placeholder aircraft representing different category/classes or models. E.g., SEL/MEL. But it didn't create placeholders for Day, night, and so forth. The idea being that you'd add any night, IMC, etc. to those flights. So if you add IMC to, say, the MEL placeholder, then that becomes MEL IMC time. The only reason to have more than one catch-up flight is to capture things that live at the model level - such as complex, category/class, tailwheel, etc. Things like night/imc/dual would not go into distinct flights.
Anyhow, editing one of these flights - even to edit the date - is simple: you can edit any flight in your logbook using the drop arrow menu on the right side of the flight in the logbook view (if you're using the website). You may have to search to find these flights. But regardless, date is an editable field.
Or did you create these flights manually (i.e., not using the MyFlightbook Starting Totals tool)? Same answer, really: find the flight and you can edit it however you like to add in whatever additional attributes you need.
And to the specific question you asked (adding a new catch-up flight with the same date as the original): Mark was correct - if you use the Starting Totals tool, you can pick the date to match the original startup flights. Or just enter your own new flight using that date.
If I'm still not answering your question, why don't we take it offline -
myflightbook@gmail.com