Mysql performance for a nullable unique id column in a large table that references a record in another table
Perhaps a cross-reference table would be the most prudent way to go.  One that has the ID from contacts, and the ID from members.  This way you know who has converted, and can keep the table as small as necessary.

As an example (I use SQL Server, not MySQL):

CREATE TABLE Members
(
    MemberID INT
    , MemberData varchar(255)
);

CREATE TABLE Contacts
(
    ContactID INT
    , ContactDate datetime
    , ContactData varchar(255)
);

CREATE TABLE MembersContactsXRef
(
    MemberID INT
    , ContactID INT
);


I would insert rows into MembersContactsXRef each time a conversion occurred for a given member.  Both fields in MembersContactsXRef could be foreign keys.