Avoiding break of cms project starting

I try to change a page property type from string to XhtmlString.

I know this gives InitializationException for the CMS (in some cases e.g. Pages and not Blocks).

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InvalidCastException: Unable to cast object of type 'System.String' to type 'EPiServer.Core.XhtmlString'

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Is there anyway to force cms to start up and ignore InvalidCastException?

We are on Optimizely 12

Br. Zuf