Good strategy to Sell some and re-buy at a dip to acquire more shares?

I started investing several years ago and bought some large cap stocks. I've been holding them long term. I would like to grow my portfolio and accumulate more shares.

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Recently I had a conversation with a friend about this. He was saying for example:

– If you have 100 shares of AAPL and bought it at $100 (holding long term)

– Then I can take some profits, by selling off like 20 shares at some price like $170

– Now that I have more money, I can re-buy AAPL at a lower entry point and accumulate more shares.

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I am having a hard time accepting his idea.

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So would it make sense for me to sell SOME of my shares that I've held long term, and then re-buy more shares at a dip or at a lower entry point?

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Or is it better to just hold long term instead and if I have some available funds and buy the dip?