I hope you do understand the concept of block (BlockTablerecord) and block reference (BlockReference), AttributeDefinition and AttributeReference.
Yes, you can change properties of an AttributeReference in a BlockReference, because AttributeReference is not "hard" reference of AttributeDefinition in a BlockTableRecord.
However, for the other entities you see in a Blockreference, which are defined in a BlockTableRecord (text, be it DBText or MText, in your case), are "hard" reference to the block definition. They are not individual entities that you can manipulate. For the change you want to do, you have to redifine the block definition (BlockTableRecord). Then, all references (blocks user see in the drawing) would also be changed.
Well, you may define the block as dynamic block (since you mentioned dynamic property), so that you can have different text with different height stacking in the same location and set the visibility property so that only one is visible at a time. This way, you can then later "change" the text height via your code by changing visibility of the dynamic property. By using dynamic block, AutoCAD does the same thing as redifine block definition behind the scene.
So, to achieve what you want, you either redefine the block through your code, or you use a properly defined dynamic block and update its dynamic property through your code.