More questions

All bitcoins solve the same problem but with increasing levels of difficulty , is at correct?

If I mine a bitcoin today that barely solves the problem, I should immediately put it in a block and take ownship.  But if I mine a bitcoin that solves the problem by a wide margin, I can sit on it because even when the difficulty goes up my coin will still be valid.  Is that correct?

Another way to put it is I could mine a coin that would have been a  slid bitcoin 6 months ago, but because the bar has been raised, it is not a bit coin today.  Today it is worthless.