If your Yashica is in full working order I would continue shooting with it. A Rolleicord Vb or Rolleiflex T won´t give you much increase in picture quality, if any at all and you probably need to have them overhauled before you can use them. A Rolleiflex with Planar or Xenotar will possibly deliver sharper images but only at full aperture. From F8 onwards there will be hardly any difference visible. I would only upgrade if you are not satisfied with the pictures the Yashica delivers (but mind then, Rolleis will be only marginally better) or if it fails mechanically beyond repair. If you should want to buy a Rollei nonetheless, better bring some patience. It is advisable to wait for one in nice condition or even recently serviced. Otherwise you should plan in another 100-150 usd to have it CLAd, if necessity should arise.