set autoddl off;
recreate table test(id int);
commit;

set term ^;
execute block as
begin
  execute statement 'drop domain dmxx'; 
when any do begin end
end
^
set term ;^
commit;

create domain dmxx varchar(15) character set none;
--create domain dmxx varchar(16) character set none;
commit;

alter table test add s01 varchar(8) character set utf8;
commit;
insert into test(id, s01) values( 1, 'qwertyui');
insert into test(id, s01) values( 2, 'âêûėżöé');
insert into test(id, s01) values( 3, 'öüčřžéèê');
commit;
select id, s01, char_length(s01) s01_char_length, octet_length(s01) s01_octecs from test;

alter table test alter column s01 type dmxx;
commit;
show table test;

select * from test;

          ID S01      S01_CHAR_LENGTH   S01_OCTECS 
============ ======== =============== ============ 
           1 qwertyui               8            8 
           2 âêûėżöé                7           14 
           3 öüčřžéèê               8           16 

ID                              INTEGER Nullable 
S01                             (DMXX) VARCHAR(15) Nullable 

          ID S01             
============ =============== 
           1 qwertyui        
           2 âêûėżöé  
Statement failed, SQLSTATE = 22001
arithmetic exception, numeric overflow, or string truncation
-string right truncation
-expected length 15, actual 32

ISQL Version: WI-T3.0.0.31756 Firebird 3.0 Beta 2
Server version:
Firebird/Windows/Intel/i386 (access method), version "WI-T3.0.0.31756 Firebird 3.0 Beta 2"
Firebird/Windows/Intel/i386 (remote server), version "WI-T3.0.0.31756 Firebird 3.0 Beta 2/tcp (csprog)/P13"
Firebird/Windows/Intel/i386 (remote interface), version "WI-T3.0.0.31756 Firebird 3.0 Beta 2/tcp (csprog)/P13"
on disk structure version 12.0