There's at least one recent case in which the NFL flexed in a Sunday night game that was a divisional matchup that they'd already had earlier in the season. In 2011, the Week 14 Giants @ Cowboys was on Sunday night, and then just three weeks later on Week 17, Cowboys @ Giants was flexed into the Sunday night slot.
Of course, the NFL's options for that Week 17 Sunday night game are always very limited, as they need to figure out which, if any, games will have playoff implications regardless of the outcomes of the Sunday afternoon games earlier that day. So they may not have had any good alternatives in that case.
In general though, they've become increasingly reluctant to flex out the tentative game for SNF unless it's a real dog. Week 17 always being the exception because there is no tentative Sunday night game.