In[]:=
rules={
z

->x,
n_z

->
n
x
};
In[]:=
s=TrigToExp[Cosh[z]]
Out[]=
-z

2
+
z

2
In[]:=
s/.rules
Out[]=
1
2x
+
x
2
In[]:=
t=TrigToExp[Cosh[5z]]
Out[]=
-5z

2
+
5z

2
In[]:=
t/.rules
Out[]=
1
2
5
x
+
5
x
2