Any side-effects of upgrading to HD wallet in relationship to hard forks I have a very old wallet that I created about 7.5 years ago. I realize that I need to upgrade to a BIP32 (HD) wallet. I am up to date with the latest Bitcoin Core and have kept it relatively up-to-date throughout the years. My question isn't how to backup or upgrade my wallet. 

Would/could there be any side-effects of upgrading my wallet to an HD wallet BEFORE I claim my BCH/BSV/BTG coins? I would like to understand the exact specifics on what happens when I upgrade my wallet and any impacts it might have (or none) before I upgrade.
There will be no effect on those other chains since they are now independent of each other, the private key corresponding to your previous UTXO will still hold the same amount of coins at the time of their fork from bitcoin core.

Here is what happen when you run -upgradewallet from: https://bitcoin.org/en/release/v0.17.0#upgrading-non-hd-wallets-to-hd-wallets


  Upgrading non-HD wallets to HD wallets
  Since Bitcoin Core 0.13.0, creating new BIP 32 Hierarchical Deterministic wallets has been supported by Bitcoin Core but old non-HD wallets could not be upgraded to HD. Now non-HD wallets can be upgraded to HD using the -upgradewallet command line option. This upgrade will result in the all keys in the keypool being marked as used and a new keypool generated. A new backup must be made when this upgrade is performed.
  
  Additionally, -upgradewallet can be used to upgraded from a non-split
  HD chain (all keys generated with m/0'/0'/i') to a split HD chain
  (receiving keys generated with 'm/0'/0'/i' and change keys generated
  with m'/0'/1'/i'). When this upgrade occurs, all keys already in the
  keypool will remain in the keypool to be used until all keys from
  before the upgrade are exhausted. This is to avoid issues with backups
  and downgrades when some keys may come from the change key keypool.
  Users can begin using the new split HD chain keypools by using the
  newkeypool RPC to mark all keys in the keypool as used and begin using
  a new keypool generated from the split HD chain.


For a proper way to upgrade to an hd wallet check How do I migrate my Bitcoin Core wallet to an HD wallet?