How to replace aris rail in existing concrete posts ?

Whats the best way (or any way) to replace these aris rails in existing concrete posts ?
The posts have oblong holes in them into which the aris rails are fitted, presumable when installed it was end post, aris rail, feed next post onto rail and fix post, more rail and so on to the end of the fence.

Any suggestion on how to replace the rails, obviously, without having to take the posts out.

Many thanks in advance.

jobby - you bend them like a longbow ... fit one end, move along the rail 2/3rds the way, one arm hook over the rail the other arm hold the rail near the end, bend the rail, guide the rail to the mortice hole with the holding hand, let go all, the rail will 'spring' into the hole. Method has worked for me for years. Tip ... do a dry run to check that the rail tenons fit into the mortices before you do the Robin Hood impersonation. The above is easier to do with 2 people.

if necessary you can shave a little of the length of one end to make them easier to get in as you do the above method.

Thanks guys, it had just not occurred to me that they would have that much flex, but I guess being half timbers they obviously do. Many thanks again.