What version of Mathcad are you using? Can you post an example worksheet showing what you mean / want to do?
If you are using Mathcad 15 or earlier, then I suspect the lines you are referring to are the printed page boundary markers and should reflect the paper size of your selected printer - if you change the paper size in Page Setup then the lines will shift to accomodate the new size. They serve purely to demarcate how the worksheet will look when printed out and have no functional purpose (ie, they are not 'section' boundaries).
You are correct in that Mathcad evaluates a worksheet left-to-right, top-to-bottom. Once you define a variable, its value remains in force from that point onwards unless you redefine it. A redefinition applies from that point onwards; you may redefine a variable as any type. Importantly, redefinitions do not back propagate up the worksheet. Two exceptions to this are globally defined variables and locally defined variables.
Mathcad makes two passes through a worksheet when it evaluates it. In the first pass, it looks for any definition made using the global definition operator (the triple bar as opposed to the := operator). It then makes a second pass looking for the normal := definition operators and global operators. Consequently, you could globally define a variable at the bottom of your worksheet and it will be in force for 'normal' calculations at the top of the worksheet ... however, if you then redefine the variable using := before the global definition then the new value will be effective from that point on until the global definition.
Locally defined variables (left arrow) are normally used in programs. They are, as the name implies, only valid within the program and behave like normal programming variable, eg you can define a variable before a loop then have the program make changes to the variable at the end of a loop that will then remain in effect at the next iteration of the loop.
So, if you've managed to stay with me so far, you can't rearrange the worksheet to make Mathcad forget about previous definitions. However, it is not normally a major problem for writing worksheets and there are some truly complex and monstrous worksheets out there in the wild. It may be that your 'problem' can be addressed in other ways that make better use of Mathcad's structure and capabilities.
Stuart
