HiI need a lisp routine to replace block with non block object.....such as closed polyline or mpolygon, and preserve block's object data table.I know it is possible to replace block with another block and preserve original OD ("ReplaceBlock.vlx" routine, found on this forum.)Is it possible?

What should be the purpose of such replacement? Do you mean putting e.g. a hexagon in place of the former block reference? What is the difference to putting in a block containing a hexagon?

What do you mean by "preserve block's object data table"?  Part of that is the mere fact that it's a Block, which it can't be any more if you replace it with a non-Block object.  Another part of that is an insertion point, which could possibly be relevant to its replacement, depending on what kind of object it is, but if it's a Polyline, the obvious entry would be for its first vertex, which may have no meaningful relationship to the Block's insertion point.  Other parts that probably will not be relevant are scale factors, rotation angle, and Block name.  So what about it do you hope to "preserve"?