A training issue of good vs bad

[Dr Robert Young]

I have verified that the installation I am taking over has Spamssassin 
2.63. It was trained (I presume) on good & bad emails about 1-2 yrs 
ago. SPAM has been "automatically" collected from users and fed back 
into the system. However, there has been no updating on the "good" 
email training.

Over time, more and more spam has been getting through. Would "not" 
updating the "good" email training cause such an effect to be seen?