I want to buy the new DLC, The Common Sense expansion pack. But I'm worried that my current ironman save with conquest of paradise expansion may have compatibility issues when I enable a new DLC. Would my ironman save and other saves be updated with the DLC and enable me to play the new features without any problems?
I'm afraid all old saves previous to the 1.12 patch (which will be released in line with the Common Sense DLC) will be broken and unplayable.
Apparently this patch will be quite a large overhaul. To play you're old saved games you're probably going to have to elect to run EU4 in a previous version within Steam. Or set it to not Auto update.
But then because you're running in an older version all the nice new features in the new Dlc will be unavailable to you.
Usually the act of enabling a Dlc will not disable an Iron Man save. However the larger Dlc's are usually released along side large patches. It's the act of upgrading to these new patches which can and often do break old saves. Dlc's which have already been released and minor Dlc's such as music / flag / graphics packs aren't an issue.
However as a side note the use of certain Dlc's are known make Iron Man mode saves ineligible for steam achievements. Such as character designer in Crusader Kings 2.