David is on the right path but I don't believe reflection overwrite is necessary here.
Change the type of UmbracoMembershipProvider
but leave the name the same.
<membership defaultProvider="TechBureauMembershipProvider" userIsOnlineTimeWindow="2880">
<providers>
<clear/>
<add name="UmbracoMembershipProvider" type="TechBureau.Web.providers.TechBureauMembershipProvider" enablePasswordRetrieval="false" enablePasswordReset="false" requiresQuestionAndAnswer="false" defaultMemberTypeAlias="Member" passwordFormat="Hashed"/>
<add name="UsersMembershipProvider" type="umbraco.providers.UsersMembershipProvider" enablePasswordRetrieval="false" enablePasswordReset="false" requiresQuestionAndAnswer="false" passwordFormat="Hashed"/>