First of all … the DC dictionaries are named "dynamic_attributes" not "D_A".
Secondly, … changing the "lenx" attribute in the definition will change all of it’s instances. If you need to only change a single instance, (and the definition has more than 1 instance,) then you would set the attribute in the instance’s dynamic dictionary. But there are likely also other attributes that may need to exist or be changed. (There is more discussion in the DC category on this.)
See this topic …