Cox increased the price of my plan as well as the speeds that came with my plan without informing me.

The plan went from 300/30, to 500/10. So if they were to adjust the price for downgrading from 30 to 10, then the adjustment from 300 - 500 would increase the price back to where it was, if not more. There was never a 300/10 plan. 

The plan went from 300/30, to 500/10.

If this transition from 300/30 to 500/10 "took place a year or 2 ago," the 300/30 plan is still on their website even after Cox recently adding the 2 Gig Package a few months ago.  Cox knows its out there.

the adjustment from 300 - 500 would increase the price

This would be correct because it would even out:  lower up but higher down.  However, Cox would not increase the "transitioning" price because they'd lose money.  If Cox increased my cost by forcing me onto a higher plan, I'd just lower my plan to the 250.  Its down would be 50 Mbps lower but the up would be the same.  It's money sense.

There was never a 300/10 plan. 

Correct.  There was never a 300/10 plan; however, Cox did offered it as an optional plan for subscribers wanting to remain at 300 down but with its now lowered 10 up.  According to Tiffany, "Those that met the requirements were able to opt in to keep the 300 Mbps down and 10 Mbps up."  It was never a structured plan, but it was an optional plan.

forums.cox.com/.../128001

Incorrect, people could opt out of the 500/10 plan at the time of it's inception, and go back to the 300/30 plan.  That way they could maintain their 30 upload! There was never a 300/10.  Even if the web site still shows a 300/30 plan, it cannot be implemented on any account. The system will not allow it, as it's not a valid selection. I know. If Tiffany, (whom I know personally), said 300 down/10 up, then she mistyped, and meant to put in 30 UP. 

then she mistyped

You may be correct, Tiffany may be incorrect or vice versa.  However, a Cox employee wrote it so it's all I have to reference.

My takeaway on the other post is 10 Mbps wasn't enough, but 10 Mbps is all you're going to get.  If you didn't want to pay for the 500/10 plan...albeit at a "grandfathered" price...you could "meet the requirements" and pay for the now lowered and cheaper 300/10 plan.

If the 300 is absolutely gone, the 250 plan is a lot closer to 300 than the 300 is to the 500.  It's 20% less vs 66% more.  If 300 was meeting your demands, 250 probably would as well.