While I find nothing in the rulings on Gatherer that ends the ambiguity of this scenario, I did find something in the Comp Rules that seems to end the ambiguity of this scenario.
Comprehensive Rules wrote:
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the
next time a player would receive priority. See rule 116, “Timing and Priority.” The ability becomes
the topmost object on the stack. It has the text of the ability that created it, and no other
characteristics. It remains on the stack until it’s countered, it resolves, a rule causes it to be removed
from the stack, or an effect moves it elsewhere.
Emphasis mine.
This wording in the Comp Rules would seem to support the idea that changing the text of a permanent after one of its abilities has triggered will change the text of the triggered ability. However, do note that if the permanent is destroyed before the text-changing spell resolves, last known information (LKI) will be used and the ability will still damage that player if they control Swamps. If the permanent is destroyed after the text-changing spell resolves, it will damage that player if they control lands with whatever basic land type you chose to replace Swamp with (because LKI was that the permanent's text had been changed such that each instance of Swamp became {other basic land type}).