I'm just learning about networking, and have a few questions regarding packet forwarding.
When a packet is traveling through a network to reach a destination host, does the source and destination IP addresses in the packet change? More specifically, does each router that grabs the packet, update the source IP address to itself before forwarding it? If so, how does the destination host send an acknowledgement back when it doesn't have the original source host IP address?
Thank you all in advance.
For TCP/IP protocols:
In brief, no.
When a packet is traveling through a network to reach a destination host, does the source and destination IP addresses in the packet change?
No. As mentioned in the comments, a NAT will change the addresses, but this is a technology built on top of the core functionality. If you want a through understanding of how modern internet works, you should learn this as well. But I'll address only normal aspects in this answer.
More specifically, does each router that grabs the packet, update the source IP address to itself before forwarding it?
As the data moves from the individual networks, the physical layer has its own local addressing. What you are describing is more how NAT works, but that's why it can't scale to a global network.
If so, how does the destination host send an acknowledgement back when it doesn't have the original source host IP address?
The destination host can return datagrams (~packets) because the source and destination are not modified. That's precisely how it knows where to reply.