Changing Type Array in Mongodb - arrays

Question

I mistakenly created a variable as an array and would like to remove the first element of the array and change the array type to simple strings.
I have been playing around with changing types in mongo from integer to strings and using the $set to change strings to arrays, however I am unable to find a solution to this variation.
The following is the a piece of the document:
db.test.find({"ids":"XXX"}, {"accountability":1}).pretty()
{
"_id" : ObjectId("524c26006f73207e0d977522"),
"accountability" : [
{
"environment" : null,
"performance" : null,
}
]
}
However when I tried to index it, the following appears:
db.test.find({"ids":"XXX"}, {"accountability.performance":1}).pretty()
{
"_id" : ObjectId("524c26006f73207e0d977522"),
"institution" : {
"accountability" : [
{
"performance" : null
}
]
}
}
db.test.find({"ids":"XXX"}, {"institution.accountability.0":1}).pretty()
{
"_id" : ObjectId("52435f0f6f73205f7d37a2b0"),
"accountability" : {
}
}
And lastly:
db.test.find({"ids":"XXX"}, {"accountability.0.performance":1}).pretty()
{
"_id" : ObjectId("524c26006f73207e0d977522"),
"accountability" : [
{
}
]
}
I would appreciate any advice!
Thank you!