diff --git a/src/cchdo/hydro/core.py b/src/cchdo/hydro/core.py
index 8492616..498ce21 100644
--- a/src/cchdo/hydro/core.py
+++ b/src/cchdo/hydro/core.py
@@ -1,6 +1,7 @@
 """Core operations on a CCHDO CF/netCDF file."""
 
 from collections.abc import Hashable
+from typing import Any
 
 import numpy as np
 import numpy.typing as npt
@@ -20,7 +21,8 @@
     ExchangeFlag,
     ExchangeSampleFlag,
 )
-from cchdo.hydro.types import FileType
+from cchdo.hydro.rename import rename_with_bookkeeping
+from cchdo.hydro.types import FileType, PrecisionSourceType
 from cchdo.hydro.utils import (
     add_cdom_coordinate,
     add_geometry_var,
@@ -40,6 +42,27 @@
 }
 
 
+def _filter_nones(dict_: dict[Any, Any]) -> dict[Any, Any]:
+    return {k: v for k, v in dict_.items() if v is not None}
+
+
+def var_to_whpnames(var: xr.Variable) -> list[WHPName]:
+    params: list[WHPName] = []
+    param = var.attrs.get("whp_name")
+    unit = var.attrs.get("whp_unit")
+    if isinstance(param, list):
+        params.extend(WHPNames[(combined, unit)] for combined in param)
+    else:
+        try:
+            error = WHPNames[(param, unit)]
+            if error.error_col:
+                return []
+        except KeyError:
+            pass
+        params.append(WHPNames[(param, unit)])
+    return params
+
+
 def dataarray_factory(
     param: WHPName,
     ctype="data",
@@ -290,6 +313,73 @@ def add_param(
     return _ds
 
 
+def change_params(
+    ds: xr.Dataset,
+    params: dict[WHPName | str, WHPName | str],
+    *,
+    precision_source: PrecisionSourceType | None = None,
+) -> xr.Dataset:
+    """Change the parameter identity of variables"""
+    whp_params: dict[WHPName, WHPName] = {}
+    for key, value in params.items():
+        key_ = WHPNames[key]
+        value_ = WHPNames[value]
+        if key_.scope != value_.scope:
+            raise ValueError(
+                f"From and to params must have the same scope: {key_} and {value_} have scopes {key_.scope} and {value_.scope}"
+            )
+        if key_.flag_w != value_.flag_w:
+            raise NotImplementedError(
+                "Cannot change ctd to discrete types (or reversed) yet"
+            )
+
+        whp_params[key_] = value_
+
+    # TODO: this list needs to be deterministic or somehow guaranteed by the CCHDO data model
+    # There is interplay between hydro and the params module
+    # We do want to pass though basically anything _unknown_.
+    retain_attrs = {
+        "C_format": None,
+        "C_format_source": None,
+        "ancillary_variables": None,
+    }
+    drop_attrs = {
+        "whp_name": None,
+        "standard_name": None,
+        "units": None,
+        "whp_units": None,
+    }
+
+    rename_vars = {}
+    new_attrs = {}
+    for old, new in whp_params.items():
+        # TODO make a get_by_whpname method on cchdo accessor?
+        if old.full_nc_name not in ds:
+            continue
+        old_var = ds[old.full_nc_name]
+        existing_attrs = _filter_nones({**old_var.attrs, **drop_attrs})
+        incoming_attrs = _filter_nones({**new.get_nc_attrs(), **retain_attrs})
+        updated_attrs = {**existing_attrs, **incoming_attrs}
+        rename_vars[old.full_nc_name] = new.full_nc_name
+        new_attrs[old.full_nc_name] = updated_attrs
+
+        # I just cannot seem to think of a better/generic way of doing this...
+        if old.nc_name_error in old_var.attrs.get("ancillary_variables", ""):
+            pass
+        if old.nc_name_flag in old_var.attrs.get("ancillary_variables", ""):
+            rename_vars[old.nc_name_flag] = new.nc_name_flag
+            # TODO: Flags attrs...
+
+    _ds = ds.copy()
+    for varname, attrs in new_attrs.items():
+        _ds[varname].attrs = attrs
+
+    _ds = rename_with_bookkeeping(_ds, rename_vars, ["ancillary_variables"])
+
+    check_ancillary_variables(_ds)
+    return _ds
+
+
 def add_profile_level(ds: xr.Dataset, idx, levels) -> xr.Dataset:
     return ds
 
diff --git a/src/cchdo/hydro/tests/test_core_ops.py b/src/cchdo/hydro/tests/test_core_ops.py
index b246fac..222cfdc 100644
--- a/src/cchdo/hydro/tests/test_core_ops.py
+++ b/src/cchdo/hydro/tests/test_core_ops.py
@@ -350,3 +350,34 @@ def test_add_param_cdom():
 
     testing_ds_param = core.add_param(ds_param_325, WHPNames["CDOM300 [/METER]"])
     xr.testing.assert_identical(ds_param_cdom, testing_ds_param)
+
+
+def test_change_param():
+    params = (
+        "PH",
+        "PH_FLAG_W",
+        "PH_TMP",
+    )
+    new_params = (
+        "PH_TOT",
+        "PH_TOT_FLAG_W",
+        "PH_TMP",
+    )
+    units = ("", "", "DEG C")
+    data = (
+        "7.1234",
+        "2",
+        "20.0",
+    )
+    ds = read_exchange(
+        io.BytesIO(simple_bottle_exchange(params=params, units=units, data=data)),
+        precision_source="database",
+    )
+    ds_expected = read_exchange(
+        io.BytesIO(simple_bottle_exchange(params=new_params, units=units, data=data)),
+        precision_source="database",
+    )
+
+    result = core.change_params(ds, {"PH": "PH_TOT"})
+
+    xr.testing.assert_identical(result, ds_expected)