A process for preparing modified polymer by withdrawing a slip stream of polymer melt from the discharge line of a continuous polymerization reactor, admixing in a highly modified polymeric additive into the polymer melt within the slip stream, then introducing the modifier containing slip stream late in the manufacturing process prior to the slip stream withdrawal point. The improved processes of the invention have particular utility for large-scale, continuous reactor where transitions and short production runs are economically prohibitive thereby limiting the product breath. The process is particularly suited for producing a family of copolyesters using a continuous melt phase production process.