Change a source alias during reindex on the background

[Bican (Radovan Bičiště)]

Hi,
I need help with the following scenario.
Let's say we have following indices and aliases.
index A - has an alias C and holds data
index B - has no alias and has no data
I start reindex and define the source index to be an alias. I tell to Javascript client NOT to wait for completion.
Something like:
{
"source": {
"index": "C"
},
"dest": {
"index": "B"
}
}
My reindex is running but I'm switching my alias C to the index B already.
Is it going to reindex all data from the index A to the index B?
Thanks,
Radovan