Fully understand it’s not a coding bug but perhaps a design bug - or a new feature request.
My training intervals are defined as % of max pace. Whenever my settings change, e.g. Power/Heart Rate Settings are modified, my historical workouts get recalculated, and sometimes it leads to showing unrealistic interval targets. Ask is to “freeze” calculated pace targets and show them regardless of what current % is. Alternatively, keep history of profile updates and apply effective numbers to appropriate intervals. Thanks for considering!
It´s neither a bug or design problem. What you´re asking for is baked in from the beginning 
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Every activity is stored with zones, ftp, LTHR, zone settings…
BUT, when you change anything in zones and hit the Update Activities button, all activities get your new zone settings (not ftp, threshold pace, LTHR - those remain what they were)
When you update any setting, don´t click that button. In stead, just change the settings which will be applied for future activities. If you want the activity that gave the new results to reflect those settings, open the activity, go to the Actions menu and change only that last activity to reflect the new zone settings.
If you have some historical data, you can set things straight again by using Activity List view and set the correct metrics for date ranges.
Ahha - thanks so much! All clear, so this is my learning curve 
Lesson learnt