Agreed that the likely inhibitor for OP's original routing was that what he sought falls below MCT at IST. A quick check shows plenty of US-AYT options on UA / LH / TK.
It is worth a call to UA to see whether it will waive the change fee and possible fare difference for the xIST tickets. That chance is substantially improved for OP alone as a 1K. The chances that UA will do this for the other two are less than low, but again, they are free to call.
If, as is likely, UA won't waive anything, then OP will need to re-arrange the inbound to IST. As tickets seem cheap, it may even be better to abandon and cancel what he has rather than trying to change it.
As others note, this is not a connection because it is on separate tickets, so neither UA, nor LH, nor TK are likely to help.