IF Statement removing too much

[MalcolmCICWF]

I am Using the below statement to re-assign some placements we made on a specific date into another date rollup. The only problem is that it is limiting all my data to only these dates. I am not sure what I need to do add in order to keep all my other placement date in the ACCT_DATE_ADDED field as well. I have successfully moved the below dates into the 20120531 date, but now that is all there is under that date field. I am guessing I have to add an OR or another IF statement somewhere to say I would like to keep all other dates rolled up under the ACCT_DATE_ADDED field. Is it a simple fix?

load

*,
If(Tier='Quaternary' and
     MixMatch(ACCT_DATE_ADDED2,'20120601','20120607','20120620','20120627'
                         )>0,'20120531'
   ) as ACCT_DATE_ADDED