Returning the pot onto a different fire:
When the above 6 conditions are fulfilled, then it is permitted to return the food even onto a different covered fire, even if the second fire is hotter than the original one. [Thus, one may remove his food from his oven and place it onto an electric plate or Blech if all six conditions are fulfilled. Likewise, one may move a food from on top of an open fire to a Blech or electric plate if all six conditions are fulfilled.]
If on Erev Shabbos one placed a pot on top of another pot, may he later on Shabbos place the upper pot on the blech or electric plate?
According to all, if when one removed the upper pot from the flame and placed it onto the lower pot on Erev Shabbos he had fulfilled all the Chazara conditions [intention, did not let go in between] then he may place it directly on the blech on Shabbos. If the conditions of Chazara were not fulfilled, then there is a dispute amongst Poskim as to the ruling. Practically in a time of need for Oneg Shabbos one may be lenient to place it on the blech. Although it is best to place the pot on top of an upside-down pot or plate, rather than directly on the blech or electric plate.
May one move a pot that is on a Blech from one area to another?
If the entire blech has the ability to heat the food to the point of Yad Soledes it is permitted according to all. If the area that the pot is now on is Yad Soledes but does not have the ability to heat the pot to Yad Soledes, then it is only allowed to move it to an area which could heat it to Yad Soledes if one had in mind when he placed the pot there to later place it by the hotter area.  Likewise, this may only be done if the food is fully cooked. If the area that the pot was on is not Yad Soledes at all, then it is prohibited according to all to move the pot to a hotter area.
 Admur 253:14 ““When the above conditions are fulfilled then it is permitted to return it to even a different Kirah than the Kirah which it was originally removed from, even if this second Kirah is hotter than the original one.””; Rama 253:2
 Shabbos Kehalacha Vol. 1 pages 418
 The Sheivet Haleivi and Shemiras Shabbos Kihilchasa both rule that it is forbidden to do so as the upper pot is not considered to have been on a fire when Shabbos began and thus it is considered as if he is initially placing it on the fire on Shabbos. However other Poskim allow this.
 Shabbos Kehalacha Vol. 1 pages 420-422; Piskeiy Teshuvos 253:30
 So comes out from the explanation of Rav Shlomo Zalman Aurbach. However, the Igros Moshe permits this even if one did not have intention when he placed the pot there, as long as the food is fully cooked.