Most of the printing mechanism is included with the toner, so replacing the toner will give full quality.

Problems can occur if captors are dirty or wheels too used. If the printer has less than 80.000 copies (I guess..)  there should be no problems.

You might be lucky because the 4/600PS were very slow and many people replaced them as soon as possible.. I mean before 50000 copies.

PS: don't try to print 20 pages at once..