Canot assign EIP from Nat Gatway to Instance

Try this. You can create the stack with the NatGateway option, for example, then update the stack with the "none" option, and finally update it with the NatInstance option. Note that your AWS::EC2::Instance declaration is missing AMI, subnet, and other essential configurations, so you'll need to add them.

Keep in mind that if you try to switch directly from NatGateway to NatInstance or the other way around, that will fail, because CloudFormation will attempt to attach the EIP to the destination resource before the old one has been detached. You'll have to do the switch in two steps, first removing the current NAT solution by specifying "none", and then updating the stack again with the new NAT solution.

---
AWSTemplateFormatVersion: '2010-09-09'

Parameters:
  NatPlatform:
    Type: 'String'
    AllowedValues: ['NatGateway','NatInstance','none']
    Default: 'none'
  PublicSubnet:
    Type: 'AWS::EC2::Subnet::Id'

Conditions:
  IsNatGatewayRequested: !Equals [!Ref NatPlatform, 'NatGateway']
  IsNatInstanceRequested: !Equals [!Ref NatPlatform, 'NatInstance']

Resources:
  EIP:
    Type: AWS::EC2::EIP

  NatInstance:
    Type: AWS::EC2::Instance
    Condition: IsNatInstanceRequested
    Properties:
      InstanceType: 't3.nano'
      ...add the rest of the instance declaration...

  NatGateway:
    Type: AWS::EC2::NatGateway
    Condition: IsNatGatewayRequested
    Properties:
      SubnetId: !Ref PublicSubnet
      AllocationId: !GetAtt EIP.AllocationId

  EniAssociation:
    Type: AWS::EC2::EIPAssociation
    Condition: IsNatInstanceRequested
    Properties:
      AllocationId: !GetAtt EIP.AllocationId
      InstanceId: !Ref NatInstance

I doubt that's possible to accomplish in one step with CloudFormation, except perhaps with some overly complex trickery, like nested stacks.

You can do that cleanly by parameterising the stack to allow the operator to choose between deploying a NAT gateway, NAT instance, and additionally the new option of neither. Maybe make that into a String parameter with those three options. You would always declare the elastic IP in your template but provision neither a NAT gateway nor a NAT instance when the "neither" option is selected.

To switch from a NAT gateway to a NAT instance, for example, the operator would first update the stack to have "neither", leaving only the elastic IP behind. They would update the stack again and select the NAT instance to deploy it an attach to the elastic IP that is still waiting.