This is such a common predicament; you are not alone.
You’re right to be cautious. If you were to declare your feelings outright and your friend does not feel the same way, it could be very difficult to recover your friendship. That said, if he does feel the same, this could potentially open his eyes to a future with you that previously he had not thought was on the cards. What a happy-ever-after that would be.
My recommendation to you would be firstly to focus on why this change in emotion towards him happened. There may well be a reason for this that you have not yet considered – this reason could be a clue as to whether his feelings have developed too.
Secondly, I would recommend (when it is possible), that you go out of your way to spend more one-on-one time with him. Before you declare how you feel, try adding some subtle flirtation to the situation to see if he reciprocates. You have to be subtle as he has known you for ten years and will instantly recognise if your behaviour towards him has obviously changed. However, if after you test the waters he responds favourably, then perhaps make your flirtation a little more obvious.
Tread very carefully but, ultimately, I hope this goes the way that you are wishing as a ten year friendship will be the perfect foundations for a strong relationship,
I hope this helps.