What's changed since last year that caused Republicans to reverse their position on creating a Third Party?

If you're talking about the Tea Party, they are grassroots political activists who are very unhappy with both parties. The Tea Party was never intended to become a political party. Evidently the Tea Party caucus is causing problems for the Republicans in the area of the budget and US debt. The Republicans are generally not that big on budget cuts except during time of prosperity although they aren't in favor of further spending like drunken sailors.