「〜したことがある」 vs 「〜したことがあった」

Question

One of the first patterns a learner of the Japanese language will come across is:

〜したことがある

(I) have 〜 before.

わたしは中国を３回訪れたことがあります

I have visited China three times before.

However, I also occasionally come across sentences like:

以前に一度会ったことがあった。

以前、６人がかりで、注射をいやがる私を取り抑えなくてはならなかったことがありました。

With the former, they seem to be interchangeable, but the latter was originally a sentence that I wrote, but had 「あります」 changed to 「ありました」, though among other things.

However, I also came across the following similar sentence.

以前も組織の男で４０人で取り押さえねばならんほどの大騒動になった事がある。

What is the difference between these two patterns?

More concretely, what would be the difference between the following two sentences?

以前に一度会ったことがあった。

以前に一度会ったことがある。

Edit

I would also appreciate an explanation as to why the following sentence is valid.

仕事で大阪に引っ越す前、大阪を訪れたことが1度だけある。

Answer 1

以前に一度会ったことがあった/ある。To me, they seem to be saying the same thing, though I think the former is close to ‘I met him once before‘ with more emphasis on the fact and the latter is close to ‘I’ve met him once before’ with more emphasis on experience, if I venture to say the difference. I think ’一度会ったことがある’ is smoother and more natural than ’一度会ったことがあった.’ Repetition of the words ‘会った’ and ’あった’ phonetically pronounced same looks awkward and sounds somewhat abrasive to me. It's like you're trying a tongue twister.

Answer 2

以前に一度会ったことがあった。　: We had met once before.

以前に一度会ったことがある。 : We have met once before.