Check the conference regulations.
Generally conferences are for 'new' and 'upcoming' work. They usually have a long lead in time. So if you've already published something, then presenting it as it is might not be fresh enough. It would be preferable to have updated it/developed it further.
If you end up publishing something between applying to a conference and presenting at it, that is less of a problem though.
Edited to add... you might also have to take into account copyright arrangements with the journal, especially if the conference itself publishes abstracts in a journal.