Blocks
Welcome to these Forums!
I assume [not entirely clear from your wording] that by "master block" you mean a drawing file external to a drawing in which you have Inserted Blocks that you want changed when you change that other drawing.
A Block definition is internal to the drawing it is defined in, and is not affected by anything that happens outside that drawing. But you can do it differently, in the way that is affected by changes to an external source drawing. That's exactly what an EXTERNAL REFERENCE [XREF] is for. If you put your [symbol, or building plan, or piece of furniture, or whatever it is] into a drawing as an Xref, and not as a Block, that is, with the XREF rather than the INSERT command, then it will be updated whenever the external source drawing is changed [requiring reloading if it's changed while you're working in the target drawing].
If you have some reason to need them to be Blocks rather than Xrefs, you can update a Block's definition to match an external drawing, if that has changed, by using -INSERT [with the hyphen, so it doesn't use the dialog box], and at the Block-name prompt, type in:
BlockName=DrawingName
with the equal sign, or, if the drawing name is the same as the Block name, simply:
BlockName=
If you already have Block(s) by that name in the drawing, it will ask whether you want to redefine them -- answer Yes. Once you've gotten that far, the new definition will have been brought in, and already-inserted Blocks will have been updated, and if you are simply updating the definition and don't need to add another, you can cancel the Insert command.
If, on the other hand, you're talking about changing a Block definition internal to a drawing, with BEDIT or REFEDIT, and Insertions of it are not changing, it may only require a REGENeration.
Kent Cooper, AIA 
