Alter Table Column, Which Is Part Of Foreign Key

In MS SQL Server, I have the following tables with some data in it.

create table table1 (
column1 varchar(32),
column2 int not null,
column10 varchar(255),
.....
primary key (column1, column2),
);

create table table2 (
column1 varchar(32),
column2 int not null,
column20 varchar(255) not null,
....
foreign key (column1, column2) references table1(column1, column2)
);

Now, I need to change the all column types from varchar to nvarchar to
support internationalized character set.
I am able to do so, for columns column10 in table1 and column20 of
table2 without any problems by using command:

"alter table table1 alter column column10 nvarchar(255);"

But, when I try the similar thing for column1 of table1/table2, am
getting one error message saying like: "ALTER TABLE ALTER COLUMN failed
because one or more objects access this column". I guess, this is
coming because of foreign key relationship between the tables.

NOTE: While defining the table2, for foreign key I have not specified
anything like "on update cascase" ...etc.

How can I resolve this issue? Any suggestions/solutions are really
helpful to me. Thanks in advance.