To clarify, are you asking if the old yeast cake/trub will be detrimental to the wort you're pouring onto it?
- how long did your last beer sit on it?
- did you rack the Belgian Strong off and then rack the BDN onto it right away?
If you used it right away, then there should be no issue. The BS prolly didn't have an excessive hop schedule, nor did it have trub levels that would cause a problem (guessing).