Problem about replacing syms with some other string

Here's my problem about replacing syms with some other string

Firstly, I create the followings,

va_d_cu = {'a','b'};
va_d_ba1 = {'a(-1)','b(-1)'};
va_d_fu1 = {'a(+1)','b(+1)'};

va_d  = [va_d_cu va_d_ba1 va_d_fu1];

Then, I do the followings,

sgu_cu  = [a_cu b_cu];
sgu_ba1 = [a_ba1 b_ba1];
sgu_fu1 = [a_fu1 b_fu1];

va_sgu = [sgu_cu sgu_ba1 sgu_fu1];

Finally, I run the following commands

f = a_cu + b_ba1 + b_fu1;
ff = subs(f, va_sgu, va_d)

and 'intend' to get the following,

ff = a + b(-1) + b(+1)

However, it only gives

ff = a + b(-1) + b(1);

I know that '+' cannot be used to replace something by using 'subs'. When I tried to run,

syms c
d = c;
d = subs(d,c,'f(+1)');

It gives,

d = f(1)

and when I run this,

s yms c d = c; d = subs(d,c,'+');

error message is prompted.

In order to get 'ff = a + b(-1) + b(+1), instead of using 'find and replace' technique, what commands I should use to do so?