Important questions about changing database Collation

[abc500x500]

Hi
please help.
I change all fields collation from utf8_general_ci to utf8mb4_persian_ci but only some key fileds could not change.
will it make problems in future or i can provide a high trafic website by continuing this db?
Thanks

[abc500x500]

Also please answer this question:

for example db has this key field: key-field-01 , that this field repeat in several table. if in one of table collation of this is different, is cause problem in db?
for exam one utf8_general_ic and serveral utf8mb4_persian_ic