I have found through trial and a lot of errors that it is almost always best to improve what you got instead of going through the time, expense and risk of replanting. If you think you need to replant, do small sections each year to avoid some of the risk. A good herbicide treatment will allow a lot of existing grasses to strengthen and new ones seem to emerge from years of being in the seed bank.
I reworked a 30 acre pasture last year that was an old neglected field where the previous owner only planted winter wheat. The rest of the year he did nothing to it. When I got it, there was nothing of value in it. It was thick tall old growth plants that needed to be burned but that was not an option/ I shredded it and then sprayed the regrowth. I disked it twice and drilled in Rye grass and fertilized it. The Rye grass did well and I grazed it some and then baled it getting 105 4 x 5 1/2' rolls in late March. After that the weeds came back with a vengeance so it was sprayed again with Grazonnext. Not much happened until mid June when the grasses started returning. Now it is a mixture of native grasses and is providing some decent grazing. The disking got the good stuff in the seed bank active. I am repeating the process on another 50 acres patch this year.
