The top part of 替 looks like a 'person' or 'man' (夫) speaking (曰, modified from old forms). Below are two 'hands' (廾). Imagine a MAN using TWO HANDS to SPEAK about and then CHANGE or REPLACE something.
'Kae' (as in かえる) - Sounds like 'CAR' and 'A'. You CHANGE a flat CAR tire. Or 'KA-E', like you CHANGE a KEY for another.