OTWO: That was my understanding as well that any replacements would be chosen from older faithful ones.
However the illustrating given by Jesus at Matthew 20:1-16 seems to indicate otherwise. Also we don't know what the requirements are to be "chosen". It's pure speculation on anyone's part as to who is and is not eligible.
The Parable of the Workers in the Vineyard
1 "For the kingdom of heaven is like a landowner who went out early in the morning to hire workers for his vineyard. 2 He agreed to pay them a denarius[a] for the day and sent them into his vineyard.
3 "About nine in the morning he went out and saw others standing in the marketplace doing nothing. 4 He told them, ‘You also go and work in my vineyard, and I will pay you whatever is right.' 5 So they went.
"He went out again about noon and about three in the afternoon and did the same thing. 6 About five in the afternoon he went out and found still others standing around. He asked them, ‘Why have you been standing here all day long doing nothing?'
7 "‘Because no one has hired us,' they answered.
"He said to them, ‘You also go and work in my vineyard.'
8 "When evening came, the owner of the vineyard said to his foreman, ‘Call the workers and pay them their wages, beginning with the last ones hired and going on to the first.'
9 "The workers who were hired about five in the afternoon came and each received a denarius. 10 So when those came who were hired first, they expected to receive more. But each one of them also received a denarius. 11 When they received it, they began to grumble against the landowner. 12 ‘These who were hired last worked only one hour,' they said, ‘and you have made them equal to us who have borne the burden of the work and the heat of the day.'
13 "But he answered one of them, ‘I am not being unfair to you, friend. Didn't you agree to work for a denarius? 14 Take your pay and go. I want to give the one who was hired last the same as I gave you. 15 Don't I have the right to do what I want with my own money? Or are you envious because I am generous?'
16 "So the last will be first, and the first will be last."
So Jesus sends out workers into the vineyard early in the morning. He then sends more workers at 9 am, noon and 5 pm. When he brings them in to be paid he pays the last ones first and some become disgruntled, thinking they should receive more (based upon working longer). Jesus rebukes them by saying they agreed to work for a particular sum and it's his money to distribute as he sees fit.
The point here is the ones who worked in the vineyard for a far longer period of time were paid the same as ones who has worked a very short period of time. This would seem to indicate that years of service is not an indication of who is and is not chosen (paid).