The limit of FL370 (or whatever it is for your type) is for the pressurisation system to maintain 8,000'. If TCAS says climb, you do so! The cabin altitude will then exceed its design altitude. So what! The majority of TCAS events will command a vertical deviation of no more than -/+ 3-500' commenced at 1.25g. So we are not really talking about a big bust either. Hit the plane coming the other way though...