alter table T
  alter C type varchar(50)

alter table T
  alter C type varchar(50) character set UTF8

SQL> create database "test" default character set utf8;
SQL> create table t (c integer);
SQL> commit;
SQL> show table t;
C                               INTEGER Nullable
SQL> alter table t alter c type varchar(50) character set win1251;
SQL> commit;
SQL> show table t;
C                               VARCHAR(50) CHARACTER SET WIN1251 Nullable
SQL> show version;
ISQL Version: WI-T6.0.0.544 Firebird 6.0 9694399
Server version:
Firebird/Windows/AMD/Intel/x64 (access method), version "WI-T6.0.0.544 Firebird 6.0 9694399"
on disk structure version 14.0

execute block as
declare variable N type of column RDB$FIELDS.rdb$field_name;
declare variable l int;
begin
  for
   select F.rdb$field_name, F.rdb$character_length/*, RF.rdb$relation_name, RF.rdb$field_name */ from RDB$FIELDS F
      join rdb$relation_fields RF on RF.rdb$field_source = F.rdb$field_name
      join rdb$relations R on R.rdb$relation_name = RF.rdb$relation_name
      where F.rdb$system_flag = 0 and
            F.rdb$field_type in (37,38) and
            F.rdb$character_set_id = 0 and
            R.rdb$relation_type = 0 and
            F.rdb$field_name starting with 'RDB$'
      into :N, :L
  do
    execute statement 'alter domain '||:N||' type varchar('||:L||') character set win1251';
end

      where F.rdb$system_flag <> 1 and

alter domain '||:N||' type varchar('||:L||') character set win1251