How to update old page type properties

How do you convert old page type properties to new ones?

Say I have this class

public class HomePage : GlobalBasePage
{
[Display(Name = Global.Constants.VIDEOS,
GroupName = Global.Constants.VIDEOS
)]
[AllowedTypes(typeof(VideoFile))]
public virtual ContentArea VideoFiles { get; set; }
[Display(Name = "Heading")]
public virtual string Heading { get; set; }
}

and want to cahnge the string to XhtmlString and VideoFiles to a certain blcok

public class HomePage : GlobalBasePage
{
[Display(Name = Global.Constants.VIDEOS,
      GroupName = Global.Constants.VIDEOS
)]
[AllowedTypes(typeof(VideoAndTextBlock))]
public virtual ContentArea VideoAndTextBlocks { get; set; }


[Display( Name = "Heading")
public virtual XhtmlString Heading { get; set; }
}

If I go to episerver, log in and view this page in edit mode, these new property types do not reflect.

Why?

And what is the best way to convert them to the new types?