For the record, there is no guarantee that you will get an upgrade to C+ just because there is an open seat. Like FC upgrades, DL limits the number of C+ seats available for upgrades and if there is no C+ upgrade inventory you will not be upgraded nor will be able to pick one just because there is an open seat. For a flight booked fairly far in advance, there will generally be C+ upgrade inventory. But there are no guarantees.
I booked a DTW-LAX flight a couple weeks out once and while there were still a few open C+ seats, there was no upgrade inventory and thus I was not upgraded.
Just another sign that ComfortPlus may end up being a stepping stone to DL cutting back further on the unlimited upgrade to first class benefit for Medallions.